Add stubs to libnativewindow.so.

Add AHardwareBuffer_createFromHandle and AHardwareBuffer_getNativeHandle
apex accessible API.

This allows com.android.neuralnetworks APEX to use system
libnativewindow.so, greatly reducing its size and dependencies.

Test: Flashed pixel device & run NNAPI tests.
Bug: 140440028
Bug: 144488395
Change-Id: I3eef0b9cde4dd5c8dc5955d6ca37079cd6cf706b