Don't clobber in progress page transition in applyLoadPlan()

Also only reloadIfNeeded() instead of always reloading in showCurrentTask(), which is called twice as the gesture starts and could contribute to jank.

Test: quickly quick switch from app A to B, ensure no jumpcut back to A
Bug: 205499708
Change-Id: I516020551d3f76eb4025df848bf4c88adf5499b7