Use cache dir instead of files dir

Installed packages are deleted after installation, but if installation
fails and never retried (for instance: one large package is downloaded
successfully, then a small package is downloaded unsuccessfully due to a
network error, then the user uninstalls the app because it doesn't
work), this can still lead to storage leaks from
downloaded-but-never-installed apps. We should use `cacheDir` instead:
it will allow the system and users to clear the cache themselves.

vending-app/src/main/kotlin/com/android/vending/installer/Constants.kt

@@ -8,6 +8,6 @@ internal const val TAG = "GmsPackageInstaller"
 
 const val KEY_BYTES_DOWNLOADED = "bytes_downloaded"
 
-fun Context.packageDownloadLocation() = File(filesDir, FILE_SAVE_PATH).apply {
+fun Context.packageDownloadLocation() = File(cacheDir, FILE_SAVE_PATH).apply {
     if (!exists()) mkdir()
 }