Keep libart.so cache after HookBridge

The previous commit 5da38f6 is not
complete since `GetArt(true)` will destroy the static SandHook::ElfImg
object.

Since in the previous two commits we have postponed the initialization
of LSPlant, we can now keep the symbol cache of `libart` for the
InitArtHooker call in method `OnNativeForkSystemServerPost`. Symbol
cache works because we are using local static object in the function
`GetArt`, whose life-cycle is not constraint by the function block.

magisk-loader/src/main/jni/src/service.cpp

@@ -248,7 +248,6 @@ namespace lspd {
         auto &binder = lspd::GetLibBinder(false);
         IPCThreadState::Init(binder.get());
         lspd::GetLibBinder(true);
-        lspd::GetArt(true);
 
         LOGD("Done InitService");
     }