[ Cherry-pick of http://ag/16064434 ]
[ Merge of http://go/wvgerrit/136330 ]
This changes adds a custom debug property for changing the fallback
policy used for the system. Depending on the value set, the device
will either use a "fast" fallback (30 seconds) or "default" fallback
(~1 day with exponential backoff). Setting this property to either
"fast" or "default" will end the current fallback if it has been
triggered.
Bug: 187646550
Test: Android unit tests
Change-Id: I5271f96139c1e468242f7fa742668cc791ffcf91
8b12e5acc9