Improve Task user event logging

Bug: 79539824
Bug: 79541772

(1) Added page index of the TASK that is being launched
(2) Covers all three cases of task launch (tap, swipe down, quick scrub)
UserEvent: action:FLING direction=DOWN
UserEvent:  Source child:TASK, packageHash=-1598699687, componentHash=1952702153, pageIdx=0

UserEvent: action:DRAGDROP
UserEvent:  Source child:TASK, packageHash=-744307622, componentHash=-515832044, pageIdx=4

(3) Avoid double logging (When onStop is called, it is not logged if
the cause of onStop is APP or TASK launch)

Change-Id: Ic5db7d6a640d43bfb5cd667b49e37dd0ed127d5c