[pm] fix acquireAndRegisterNewAppId after app removal

mFirstAvailableUid becomes (the last removed app ID + 1) after an app
removal. acquireAndRegisterNewAppId should use the offset of
mFirstAvailableUid as an index, not its actual value.

Test: manual for now
BUG: 216658672
Change-Id: Ie1b9ecafeb191129320243608c7f70eb2ea5c79e