Bluetooth: clean up TestObserver

Since TearDown() stops discovering, we need to make sure any local
observer is cleaned up. Best way to do this is to move the job of
adding and removing to/from the adapter to the TestObserver class.

BUG=None
TEST=device_unittests
TBR=armansito

Review URL: https://codereview.chromium.org/194863005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256308 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256308