Offer to migrate databases and SharedPreferences.

Databases and SharedPreferences often involve multiple files under
the hood, so developers wanting to migrate them between different
storage contexts should ask us to make sure all relevant files are
migrated correctly.

This makes a best-effort attempt to recover from battery pulls
during migration, while still trying to alert developers to
conflicting files.

Bug: 26668510, 25860525
Change-Id: I9ffa3e8cb6191dfd4237b9466a081d6d77df3ba0