SurfaceTexture: attach to Dalvik when needed.

This change fixes a bug in the SurfaceTexture JNI where a thread that
the Dalvik VM was not aware of calls the onFrameAvailable callback.
When this happens the callback needs to first attach the thread to the
VM before attempting to post the onFrameAvailable event for Java code to
handle.

Change-Id: I6a5470c32611ea6f38e9167779450f50635cabd3