A2DP: Restrict MTU while using SBC middle quality

When SBC headsets report middle quality bitpool under a larger MTU, we
reduce the packet size to prevent the hardware encoder from putting too
many frames in one packet.

Bug: 188020925
Tag: #compatibility
Test: A2DP playback with SBC manually
Change-Id: I164c0c1fe37d6852718889e2946207471b26e5bd