Let launcher to provide its own OverscrollPlugin

If the local OverscrollPlugin provided by the activity is non-null and
FORCE_LOCAL_OVERSCROLL_PLUGIN is true, the local plugin object overrides
the loaded actual plugin.

Test: Manual testing flipping the flag and observing which plugin is
activated.
Bug: 150245271

Change-Id: I0cd5184628c2cb572b5e1c56d6a008dcaa8807fb