device/bluetooth: Remove BluetoothAdapter::Start|StopDiscovering.

This CL removes all remaining references to the deprecated
device::BluetoothAdapter::StartDiscovering and
device::BluetoothAdapter::StopDiscovering methods. This CL also does
a minor refactor, so that BluetoothAdapter now only tracks active
discovery sessions.

BUG=346985
TEST=Compile chrome; run device_unittests.

Review URL: https://codereview.chromium.org/188473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255809 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255809