BluetoothMidiService: Subtract header from MTU

In Android 14, the Bluetooth team changed how they handle MTU.
BluetoothMidiService should now subtract 5 from the MTU.

Bug: 308909343
Test: CTS Verifier Bluetooth MIDI tests
Change-Id: Ie6f36fc5baa0563ab3bf7600de404fe9b8eb0434