Support user event logging for drag and drop

b/30039490

Supported in this CL:
- DnD: drag from container [WORKSPACE|HOTSEAT|FOLDER|ALLAPPS|WIDGETS|DEEPSHORTCUTS]
       drag to container [HOTSEAT,WORKSPACE,FOLDER,DROPTARGETS]
- Source and target can be [FOLDER_ICON, ICON, DEEPSHORTCUT, WIDGET]
- $ adb shell setprop log.tag.UserEvent DEBUG will turn on debugging

Change-Id: I0b8b879b80e6dce85bbde6e7794f9e0677832603
(cherry picked from commit 59a23809)