Reset current task id to -1 when recents animation is finished.

After we finish the recents animation, the current task id should be correctly reset so that when the task is launched from Overview later, it goes through the flow of starting a new task instead of finishing recents animation to app. A typical example of this is (1) user swipes up from app to Overview (2) user taps on QSB (3) user goes back to Overview and launch the most recent task.

Fixes: 143377934
Test: Turn on live tile, swipe up to Overview. Tap on QSB, see QSB launching correctly. Go back to Overview, tap on the most recent app tile, make sure the app is launched correctly.

Change-Id: Ibc1c8249d9b79584495bf0cec2ae6f7debfa658f