BluetoothAudioHAL: MTU not exceed an AVDTP packet

Fix the MTU value not to be greater than an AVDTP packet, so the data
encoded by A2DP hardware encoder can be fitted into one AVDTP packet
without fragmented.

Bug: 177205770
Tag: #compatibility
Test: A2DP playback and check the MTU that Audio HAL receiving
Change-Id: I9104b699448b55fb2ec981aecb6ce1913d494821