Bluetooth: minor documentation fix to de-confuse

It was somewhat unclear which is the preferred method of retrieving a
BluetoothAdapter.  Make it clear that using BluetoothManager is
preferred, and remove references to the old method in BluetoothManager
docs since it is only avaialable in API 18 or higher.

Test: recompile, check that documentation is updated
Fix: 33355430
Change-Id: Ia20b4e45dca03bc2f13c2ab477799b89c5e14f45