Support toggling Taskbar All Apps with 3P Launcher.

Taskbar All Apps exists regardless of the default launcher. Thus, we can
toggle it on large screen devices. This CL ties registering the system
action to default launcher and taskbar's enablement.

Test: adb shell input keyevent 117
Test: AllAppsActionManagerTest
Flag: LEGACY ENABLE_ALL_APPS_SEARCH_IN_TASKBAR ENABLED
Fix: 317259709
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c113b277e61eb0391f050d6c12bf36711d727733)
Merged-In: I26f0ed9e921beac762f3f9e6aaceb1002ad4801a
Change-Id: I26f0ed9e921beac762f3f9e6aaceb1002ad4801a