Fix ACameraMetadata_fromCameraMetadata crash.

Root cause was that jclass instances acquired from JNIEnv::FindClass are
not pinned and become invalidated due to Java garbage collection. Adding
a new global ref pins the pointer.

FIXED=148174094

Test: atest android.hardware.camera2.cts.CameraManagerTest#testCameraCharacteristicsNdkFromSdk android.hardware.camera2.cts.CaptureResultTest#testCameraCaptureResultAllKeys
Test: atest NativeCameraDeviceTest NativeCameraManagerTest NativeImageReaderTest NativeStillCaptureTest

Change-Id: Id0601b9c01e1a58485e3b039a87a5cf56a19e0af