BLE-MIDI: fix timestamps for sysex data

Blocks of data in the middle of a SysEx used to get a zero timestamp.
Now they get a proper timestamp.

Thanks to Teenage Engineering for suggesting this fix.

I also cleaned up the code to make it easier to understand.

Bug: 140638458
Test: Send long, >50 byte, SysEx over BLE to an Android
Test: device running the MIDI scope. Check timestamps
Test: for the data blocks. See bug for more details.
Change-Id: I1a553d54cc76025f854e53229d233a033ab7c297