Add API to get remaining lifetime as a percentage.

This differs slightly from the previous API, which exists for idle
maintenance, whereas this value is intended to be displayed to users.
First, it returns remaining lifetime, rather than used lifetime. Second,
it rounds up the returned value for usabilty purposes. This isn't an
issue on Pixel (which reports at 1% granularity), but devices which
report at 10% granularity should show 100% out-of-box, which is not
possible to distinguish in the old API.

Bug: 309886423
Test: StorageManager.getRemainingStorageLifetime
Change-Id: Ic5f6ec9969667302ba8bad95b2765e2cc740bed4