[pm] remove circular dependency in init helper

mPm.mOverlayConfig is needed during the initial non-system app scanning.
Therefore we have to assign it before the start of non-system app
scanning. This CL fixes that.

+ minor refactors

BUG: 211822574
Test: boots
Change-Id: Idff0026038f876dba5dcf434d40a8f6999a43e3c