tp: optimize min_ts_per_track query (#1428)

Optimize the min_ts_per_track query for tracks with large numbers of
samples by limiting the results of the query to only contain timestamps
for track event tracks and not power rail tracks.

This query change is essentially @LalitMaganti's suggestion from
https://github.com/google/perfetto/issues/1323 with some minor
formatting fixes.

Testing:
- Validate example Android and Chrome trace open (there is a very minor
~20 ms perf penalty from on loading these traces)
- Validate that power rail trace from
https://github.com/google/perfetto/issues/1323 now shows negligible time
spent loading plugins (now much less than 1 sec, previously ~10 sec)

Bug: https://github.com/google/perfetto/issues/1323