Make mTvSystemAudioModeSupport originally null and fix related logic

Note that before the Android device queries if the TV supports SAM, we
should set mTvSystemAudioModeSupport to null. After quried, set it to
true or false accordingly.
Also we devices goes to standby or loses its logical address, this
boolean should be reset to null

Test: manual
Change-Id: Id39dc2a9b0ee9de010268914a745a6aaae0287ea
(cherry picked from commit 05f01112f9931a058d9fcecb956c1ff35630ed0c)