Throttler for incidentd based on size putting into dropbox.

The incidentd will accumulate the total size put into dropbox and once
it exceeds a threshold (currently 20MB) daily, it will stop further
requests. It allows collection again 24 hours later.

Bug: 64219725
Test: atest incidentd_test and manually flashed incidentd and test.
Change-Id: Iea21fbae40d5d01108797b190231d73e74eff213