Permit settings to "wipe" the persistent partition

One of the requirements is that when the user does a
factory reset through settings, all data on the
persistent partition should be cleared.

This adds one last API method that allows settings
to wipe the partition.

Bug: 14288780
Change-Id: Ib87ee741d1e5195814516ae1d66eb7c4cf754dcf
4 files changed