AMS: set reset_on_fork flag for app threads in OomAdjuster.

This flag is used to reset the thread's scheduler policy to the default policy when the thread is forked. Since the priority of those threads is set and maintained by OomAdjuster, setting this flag will ensure forked threads from these managed threads start with a fixed priority.

Bug: 370988407
Flag: com.android.server.am.reset_on_fork_enabled
Test: Build and check with chrt -p [APP_PID]
Change-Id: I811800a3684a39b2270e7b0e6e15ad6159c36a9d