Allow Floating ActionModes in SearchDialog.

The current implementation was always blocking them due to the override of
the typeless startActionModeForChild method. The typed method already
supports blocking PRIMARY types and lets FLOATING through, but the super
implementation always checks the typeless implementation, hence the
FLOATING action modes were also blocked.

Also cleaned up an unused field and its setter.

Change-Id: I115b4511f1b61512b293af3e53af72c22412ccb0