[PB] Pilfer pointers if no focus window.

startBackNavigation can return null if no focus window, which can happen
if a transition is playing. We should still pilfer pointers in case the
touch event send to app once it become focused.

Flag: EXEMPT bugfix
Bug: 351763151
Test: trigger gesture right after transition, verify app won't receive
motion event.

Change-Id: I535085561ebc56e24672b859859587da349da6e8