PermissionRequest: Several updates

The app now handles runtime permissions on M and above. The runtime
permission is purposely isolated from PermissionRequest API in order to
minimize confusion.

The web app use MediaStreamTrack.stop() instead of deprecated
MediaStream.stop().

BUG: 29983168
Change-Id: I43c5f889847868e8517a5b262c724486c1f53805
(cherry picked from commit 9297fb39)