Merge remote-tracking branch 'goog/androidx-platform-dev'

Per yamasani@'s request, I have broken up ag/15113841
[I1d9d017f6d225c292295b2fbccd42ea731ef40c0] into smaller chunks to
simplify review.

This change should only include the changes for:
- Switch DocumentStore to using PortableFileBackedProtoLog instead of
FileBackedProtoLog.

I've made one additional change from the state of the relevant code
compared to ag/15113841. Per discussion with yamasani@ and adorokhine@,
we decided to skip the migration and just accept data loss given the
small population of devices that will have existing AppSearch data in
system_server. The old migration code still exists, but is behind the
ENABLE_V1_MIGRATION preprocessor macro which will not be defined.

Bug: 185806837
Test: Presubmit

Change-Id: I589fa722bf2695127a127db550113d937d5590f4