AudioService: AudioDeviceBroker: get codec in message handler

If we call mBtHelper.getA2dpCodec before msg handling, the codec could
be updated later by other msgs in queue.

Bug: 289681953
Test: make
Test: test on Pixel device with A2DP device connect/reconnect
Change-Id: Ia8d542a3a2ca9f86489e4370cca8f0eb16f26e17
(cherry picked from commit 2aceac75)
Merged-In: Ia8d542a3a2ca9f86489e4370cca8f0eb16f26e17