[res] Overlayable only mode of loading ApkAssets

OverlayManagerService only needs to know the overlayable
declarations of the apk, and as of now the only way to get those
is to completely load and parse the whole resources file.

This CL adds a flag to only parse the overlayable declarations,
speeding up loading by ~2x

Test: build + boot + UTs + user switching
Bug: 271904589
Change-Id: I1ec4b9b57dd1aee1769cc2d4dd9641e5e68639f8