Moving SysUI with legacy Recents to another build target

- Move all legacy recents code out of the core sysui code
- Add separate target for products which still depend on it
- Move overview proxy implementation into another implementation of the
  recents interface

Test: atest SystemUITests
Test: Push SystemUiWithLegacyRecents to the system image
      adb shell pm disable com.google.android.apps.nexuslauncher/com.android.quickstep.TouchInteractionService
      and ensure that the old recents implementation still works (and split
      screen)

Change-Id: Iad67218ec37c13c79b6393d87f6bdc4f3996e2c6
Signed-off-by: Winson Chung <winsonc@google.com>