Change AtomicFile to use rename-into-place.

The previous implementation of backing up beforehand doesn't handle
the case where the file is created for the first time, and might leave
a corrupted file in case of failure.

This new implementation creates a new file for writing data into, and
renames it into the place of the original file after writing
finished.

Fixes: 151959443
Test: atest android.util.AtomicFileTest
Change-Id: I5c4c438526a2aecdd2af18f71e16b41a05817c61
Merged-In: I5c4c438526a2aecdd2af18f71e16b41a05817c61