Prevent dispatching onBackStarted too early for gesture nav

Bug: 383527784
Flag: EXEMPT bugfix
Test: Manual, i.e. testing touches and long clicks in apps at screen edge on screens with predictive back system animations
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c7a24c3d9f85c68d720260a6d4892c18a3a047c8)
Merged-In: Ie596e2a39bfae6db2a574552999f5f128d4aa665
Change-Id: Ie596e2a39bfae6db2a574552999f5f128d4aa665