Always re-enable the fallback WebView if needed.

If there's no valid WebView implementation at boot time, attempt to
re-enable the fallback package, even if we already did the one-time
migration from an earlier OS version.

This may help to ensure devices don't end up with no WebView if
something went wrong with the migration.

Bug: 132815849
Test: manual
Change-Id: I9c9cf634ea0de9b52a27d4c1100d95fad6047b05