bluetoothmidiservice: fix timestamp for BLE-MIDI transmitted packets

Transmitted packets that contained multiple MIDI messages and that did not
use running status were missing a timestamp! This CL fixes that bug.

Bug: 34708799
Test: a manual test using MidiMatrix is described in the bug report
Change-Id: Iddb55e01c60625787c5907fe8e4b202ef1415e59
Signed-off-by: Phil Burk <philburk@google.com>