Make SharedPreferences check with higher precision

When two processes modify shared preferences we use the timestamp to
figure out if the file was changes underneath. Do this with the highest
precision available (instead of sec) as before.

It would be possible to make the check more reliable by writing a unique
id to the shared pref file, but this would make this check much more
expensive in the common case that nothing changed. Considering that this
has not been a problem and we don't officially give any guarantee for
this sounds like a good middle-ground.

(cherry picked from commit ffe74357)

Merged-In: I04c96b6a946618d5599c26410c88d7cd654d31fb
Change-Id: I04c96b6a946618d5599c26410c88d7cd654d31fb
Test: SharedPreferencesTest
Fixes: 62949739