Update javadoc for the API and change return of getBondedDevices().

getBondedDevices() *might* work if called before the Bluetooth
State intent is broadcasted. However, this can cause ANRs and problems.
This API was updated to return null, if called before the intent
is received. However, this might cause existing apps to crash. Return
an empty set instead.

Change-Id: Ibc484d3394aa0bbebd651221efde6a7015ce7110