AudioFocusRequest: more docs, listener without handler

Focus listener: follow platform pattern for setter of listener
  and handler (set(listener) and set(listener, handler) methods).
More docs.

Test: cts-tradefed run cts -m CtsMediaTestCases -t android.media.cts.AudioFocusTest
Bug: 30258418

Change-Id: Ib5d1fbc3cf92821b63ffc1b4ad4cf4384bb6ff54