Cache storage info into file.

System is able to query the storage info from AppSearch through
AppSearchStorageStatsAugmenter. Currently, AppSearchImpl is required to
be initialized for querying the storage info. So if system need to query
the storage info before AppSearch is connected/initialized, it would
introduce multiple AppSearchImpl instance initializations for user,
which may block the Setting-Storage UI thread.

In this CL, we cache the user's storage info into a file under their
credential encrypted system directory while initializing
AppSearchUserInstance for the user.
Next time, if system need to query the user's storage info before
AppSearch is intialized, it would try to read the cached info from the
file on disk.

Bug: 179160886
Test: UserStorageInfoTest
Change-Id: I1f760879858a4d667b734018f63e108084c54e1a