Precompute the RTP Header marker bit only for A2DP Source encoder

The RTP Header marker bit is used only by the A2DP Source encoder,
and should be computed only when the local device is A2DP Source.

Test: Manual - disconnect, reconnect A2DP headset, play Audio
Bug: 74250013
Change-Id: I6510eeb9a34397e6623e5bc36675fc833b96ad7d
(cherry picked from commit 97390c0ea9eff71c3959989142c3eaeb243837a0)