avrc: Fix AVRC_MsgReq() msg_mask bug

msg_mask is intended to be a bit field.  But by declaring it a
bool, we end up turning all bit values into 1, and losing the
other bit values.  We fix this by properly declaring this as
a uint8_t.

Bug:31273148
Change-Id: Id95e1853ea1ad4fbebc35fdc24708765f1fd9e07