Fixed some minor bugs in MotionEvent recycling.

1. MotionEvent.recycle() never set mRecycled to true so it couldn't actually
   detect doubly-recycled events (unless the TRACK_RECYCLED_LOCATION debuging
   flag was enabled).

2. MotionEvent.obtain() did not set mNext to null before returning the event
   so it would unnecessarily retain a reference to other events in the
   recycled event linked list until recycled again.

Change-Id: I93709c402d260691875f632dfc080a355f85fbb0