Implement quota calculation for project ID based quota.

Devices launching with R will no longer have the sdcardfs filesystem;
instead, quota tracking on external storage is implemented by project
IDs. Switch over the existing quota calculation to use project IDs when
sdcardfs is not available.

Bug: 146419093
Test: atest StorageHostTest (on devices with and without sdcardfs)
Change-Id: I17925c811b08c7c85fff02ee6e279e4d7586e3ff