Add feature flag to force taskbar to be persistent.

This will allow us to test taskbar pinning before the entrypoint for
such a setting is finalized.

Test: manual, TODO: will need to update TaplTestsTaskbar to account for
this mode
Flag: FORCE_PERSISTENT_TASKBAR [off]
Bug: 258604917

Change-Id: I4535978563ffbe9a6cc6ece7720cdb75e787ff44