Guard new added freezen metric by feature flag.

The main build is still be used for U release, the metrics cts
test fails because this new added metric. Guard this new change by
a feature flag.

Bug: 307561242
Test: atest PackageInstallationSessionReportedStatsTests
Change-Id: Ife56a8d2097a90ee841a9e4749e7a695e52322d1