[pm/metrics][2/n] pass snapshot to metrics when finish

Snapshot is needed to collect information about installer UID and
installed package version code.

+ minor improvements

BUG: 249294752
Test: atest com.android.cts.packagemanager.stats.host.PackageInstallationSessionReportedStatsTests
Change-Id: Ibaf94132f70bd7a8a74351a297dc92fe515258f3