[pm] use actual scanning time as firstInstallTime for preload apps

Previously we use the file modified time as the firstInstallTime for
preloaded apps. This timestamp can be much older than the actuall
boot time, and can cause ART service to skip some of the preloaded apps.
This CL changes it to use the actual scanning time of the apps as the
firstInstallTime.

This CL is guarded by a feature flag to minimize compatibility issues.

BUG: 321258605
Test: manual

Change-Id: Ic2480de9e26c8211ef1957319a1e97ddc9471278