PackageManagerService: regenerate snapshot before writing settings

Writing settings holds the PM lock for a potentially long time while doing IO. If the snapshot has already been invalidated and is pending recreation, then any reader of a snapshot has to wait for that IO to finish. Regenerate the snapshot whenever necessary before writing settings.

Test: boot trace + reduced PM contention
Bug: 270746433
Change-Id: I3ad2dead54a0ac79baecad53b13c0394028bf612