Add queryStatsForPackage() API.

This gives developers a way to collect package-level stats, even if
it means we have to use manual calculation for sharedUserId apps.

Also round size of storage devices to nice power-of-two values so we
do a better job of matching retail packaging.

Test: builds, boots
Bug: 35294241
Change-Id: I24946c443bb9dc4b0411a8149a0656702ac1fd24