build: migrate to AGP 9 built-in Kotlin support

AGP 9 provides Kotlin support natively and refuses to configure a
module that still applies org.jetbrains.kotlin.android alongside it.
Removes that plugin (all its declaration styles: catalog alias,
kotlin("android"), and hardcoded id(...)) from every module actually
wired into settings.gradle.kts, and drops the now-unused kotlin-android
catalog entry.

Also fixes two things AGP 9 surfaced as hard errors during
configuration, unrelated to the plugin removal itself:
- ApiDemos:java-app had a vestigial buildFeatures.compose = true with
  no Kotlin sources or Compose dependencies; AGP 9 now requires the
  Compose Compiler plugin whenever compose is enabled, so the dead
  flag is removed instead.
- tutorials/kotlin/Polygons used the now-unsupported
  proguard-android.txt default file, replaced with
  proguard-android-optimize.txt.

Standalone tutorial/sample directories not wired into
settings.gradle.kts (app-rx, the java/ and duplicate kotlin/ tutorial
folders) are untouched since they aren't part of the actual build
graph or CI.

Author: kikoso

ApiDemos/project/common-ui/build.gradle.kts

@@ -18,7 +18,6 @@ import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 
 plugins {
     alias(libs.plugins.android.library)
-    alias(libs.plugins.kotlin.android)
 }
 
 android {

ApiDemos/project/java-app/build.gradle.kts

@@ -34,7 +34,6 @@ android {
     }
 
     buildFeatures {
-        compose = true
         buildConfig = true
         viewBinding = true
     }

ApiDemos/project/kotlin-app/build.gradle.kts

@@ -18,7 +18,6 @@ import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 
 plugins {
     alias(libs.plugins.android.application)
-    alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.parcelize)
     alias(libs.plugins.secrets.gradle.plugin)
 }

FireMarkers/app/build.gradle.kts

@@ -22,7 +22,6 @@ plugins {
     // Compose. They handle the core compilation, packaging, and resource processing.
     // ---------------------------------------------------------------------------------------------
     alias(libs.plugins.android.application) // Applies the core Android application plugin.
-    alias(libs.plugins.kotlin.android)      // Enables Kotlin language support for Android.
     alias(libs.plugins.kotlin.compose)      // Provides support for the Jetpack Compose compiler.
 
     // ---------------------------------------------------------------------------------------------

FireMarkers/build.gradle.kts

@@ -17,7 +17,6 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 plugins {
     alias(libs.plugins.android.application) apply false
-    alias(libs.plugins.kotlin.android) apply false
     alias(libs.plugins.kotlin.compose) apply false
 
     alias(libs.plugins.hilt.android) apply false

WearOS/Wearable/build.gradle.kts

@@ -17,7 +17,6 @@
 plugins {
     id("com.android.application")
     id("com.google.android.libraries.mapsplatform.secrets-gradle-plugin")
-    kotlin("android") // The Kotlin plugin is applied differently in Kotlin DSL
 }
 
 android {

WearOS/build.gradle.kts

@@ -17,5 +17,4 @@
 plugins {
     alias(libs.plugins.android.application) apply false
     alias(libs.plugins.secrets.gradle.plugin) apply false
-    alias(libs.plugins.kotlin.android) apply false
 }

build.gradle.kts

@@ -17,7 +17,6 @@
 plugins {
     alias(libs.plugins.android.application) apply false
     alias(libs.plugins.android.library) apply false
-    alias(libs.plugins.kotlin.android) apply false
     alias(libs.plugins.kotlin.parcelize) apply false
     alias(libs.plugins.secrets.gradle.plugin) apply false
     alias(libs.plugins.hilt.android) apply false

gradle/libs.versions.toml

@@ -143,7 +143,6 @@ volley = { group = "com.android.volley", name = "volley", version.ref = "volley"
 android-application = { id = "com.android.application", version.ref = "androidGradlePlugin" }
 android-library = { id = "com.android.library", version.ref = "androidGradlePlugin" }
 hilt-android = { id = "com.google.dagger.hilt.android", version.ref = "hilt" }
-kotlin-android = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
 kotlin-compose = { id = "org.jetbrains.kotlin.plugin.compose", version.ref = "kotlin" }
 kotlin-parcelize = { id = "org.jetbrains.kotlin.plugin.parcelize", version.ref = "kotlin" }
 kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }

snippets/app-compose/build.gradle.kts

@@ -16,7 +16,6 @@
 
 plugins {
     alias(libs.plugins.android.application)
-    alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.compose)
     alias(libs.plugins.secrets.gradle.plugin)
 }