[PB] Skip predictive back if prevous activity has not created

The activities can remain in INITIALIZING state when launched with
startActivities, predictive back should not invoke to those activities
because there must have no snapshot, also make them visible will kick
lifecycle change.

Bug: 345951785
Flag: EXEMPT bugfix
Test: follow issue description
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:387b7e5765bcac8c57c55353616409ec55859d6a)
Merged-In: Ib5360c15fc68716236f3c62940971e965b46f21f
Change-Id: Ib5360c15fc68716236f3c62940971e965b46f21f