BluetoothMidi: request higher priority, for lower latency

Request a higher connection priority.
This will result in a Connection Interval below 15 msec
vs 45 msec for some devices.

Fix: 34710538
Test: Connect a BLE-MIDI device using the MIDI+BTLE test app. See bug.
Change-Id: I846a76a36c864f82969173a94d86b05059a786ad
Signed-off-by: Phil Burk <philburk@google.com>