Remove FastJNI optimization on AssetManager to avoid dead lock.

I found there was a dead lock among main, android.display and GC threads
when running monkey test.
 - Main thread got a mutex and was suspended by GC thread.
 - Android.display thread waited for mutex held by main thread.
 - GC thread waited for suspention of android.display thread.

This will lead to ANR or screen freeze.

Fixes: 32480078
Test: builds

Change-Id: I13cf1eca3cb3b7c01aa754874f2b48aab0b472e8