Sync: Improve persistence in PRE_ tests on Android

It turns out that at the end of a PRE_ test, not necessarily all data
gets persisted before the process is shut down. Specifically, I have
observed (a) prefs, (b) bookmarks, and (c) FakeServer data not being
persisted.

This CL adds explicit "flush to disk" code for these types.

This is clearly not a proper solution - there are very likely other
pieces of data that don't get reliably persisted, and it's not scalable
to try to list them all in SyncTest. But it's the best workaround we
have for now.

This will allow us to enable a few more PRE_ tests on Android (in
followup CLs).

Bug: 479828012, 467211652
Change-Id: I3c69872b752c66558cebbe42d22db7ac6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7594504
Reviewed-by: Ankush Singh <ankushkush@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1596269}