adds logging code for InitAppScanReported

Introduces the InitAppScanMetrics class for handling the stats related
to boot-time app scan performed by package manager. The class is focused
on the verification phase of the scan and logs stats such as the
duration of verification, its outcome and various APK attributes.
Stats are logged using the atom InitAppScanReported.

Adds the logging code to InstallPackageHelper to initialize the metric
and record it on per-APK basis. The metric is only recorded for updated
system apps. This is because the main goal is to evaluate the signature
verification phase, specifically for apps that are allow-listed for Full
Stack Integrity (FSI) check. System directory apps are covered by
Verified Boot (VB) not FSI, and FSI checks do not apply to non-system
apps in the data directory.

Bug: 422817705
Flag: EXEMPT Metrics
Test: statsd_testdrive -e 1171
Cq-Depend: CL:34095157
Change-Id: I5de84f5aa94b21974c9c5d8f0732155a56ef4e8b