AVRCP: Reset mActivePlayerId on cleanup

Sets mActivePlayerId to NO_ACTIVE_PLAYER on cleanup,
to avoid a NullPointerException on callbacks after cleanup.
This has been observed when AVRCP is stopped but the
media button session has just been changed. This leads
to the session callback being called on the active player,
which has just been cleaned and this is NULL.

Test: manual
Change-Id: I23aefb1f3ac47dbfdb2b934e5224f8da0f6cb8c8
Signed-off-by: Dowon Park <dowon.park@samsung.com>