Don't unfreeze too early

This change removes an unnecessary (and incorrect) unfreezing during the
install flow. Removal ensures that all state from the previous version
of the updating app has been cleaned up before unfreeze and launch of
the app.

Bug: 136731650
Test: atest AtomicInstallTests AppSecurityTests
Change-Id: I477a6c352268caf5031d85bf8bd8d322e7472f33