Fix ApkAssets mNativePtr accesses

Actually acquire the monitor it claims to be guarded by. Since it was a
final field, it's still not entirely clear what that means.

Clear mNativePtr when we deallocate its referent. This prevents native
heap corruption if methods here are called from another finalizer.
Remove mOpen and use mNativePtr instead.

This probably does not fix the bug below. But let's get rid of the
possible heap corruption source we know about.

Bug: 159041693
Test: Build and boot
Change-Id: I6f0f6250ca8b6a4274c346ae99f1f94cab5844e1