Fix double back swipe not working

The keepMinimumAlpha check is not needed anymore since the closing target always keeps alpha at 1f during the pre-commit phase.

Bug: 346481208
Flag: com.android.systemui.predictive_back_system_anims
Test: Manual, i.e. verifying that double back swipe does trigger the second transition when the first transition has ended
Change-Id: I0c9b325e8047b760cf01fd7d931e4e1e2877d0b3