First phase implementation: dragging and animation interaction is implemented
namely in two classes. ScrollGestureDetector and AllAppsTransitionController.
FeatureFlag.LAUNCHER#_ALL_APPS_PULL_UP will be true for only AOSP and
not in the extending builds. This way, we can safely iterate without
turning it on the shipped ready version.
b/28917826
Change-Id: I0501309c0121880ffe0555f82d6ac5a145581bb1
