BQR: Cleanup BQR header definititon

* Rename persist.bluetooth.bqr.eventmask to persist.bluetooth.bqr.event_mask
* Rename persist.bluetooth.bqr.interval to
  persist.bluetooth.bqr.min_interval_ms
* Rename all instances of reporting interval to minimum report interval
  milliseconds
* Add zero initialization to all event values
* Add comment on the unit of timestamp and piconet clock

Bug: 111384296
Test: make, no functional change
Change-Id: I5174297ec314a862da7591bcf3aefea9b6778f1a