Check for drmhal plugin null pointer deref

If binderDied is invoked after a plugin is destroyed
the pointer can already be null. Also reset the init check
state after the plugin is deleted to make sure the
checks in other methods remain valid.

bug:62667492

Test: Play Movies, ExoPlayer
Change-Id: Ie12191b744331a7a435b4c64bbd1c179918553ac