Add initial support for search session in Taskbar All Apps.

A search session is represented as a stub ResourceBasedOverride that can
be extended for controlling a search session.

Test: Manual (including aosp Quickstep)
Bug: 216683257
Flag: ENABLE_ALL_APPS_SEARCH_IN_TASKBAR
Change-Id: I9cf4b5f84feec5215840d456b601c6a9f7c4c211