Allow dragging to launch multiple instances of the same activity

- When dragging to launch a new app, apply MULTIPLE_TASK to the launch
  intent to allow apps that support it to show side-by-side
- This requires the pending intent resolved from LauncherAppsService to
  be mutable (should be ok since we only use this api from SysUI)
- Also remove some unused members and duplication of getting the running
  task when dragging

Bug: 207686016
Test: atest WMShellUnitTests

Change-Id: Ib233ad754a6c6e3c4e0d0e10ed788ab8e055cccc