Make BugreportStorageProvider subclass FileSystemProvider.

This allows it to gain all the benefits of FSP, ie. listening in on
File-system level changes without any explicitly notification Uri
passing back and forth.

This also reverts commit 6c9ff513c6d7d3de7ab8920d88111c2b1dc1bb1f.

Test: Manual test.
Bug: 38183534
Change-Id: I30a1d8489ec8732bdb80611bd8fa7099763de2a3