Fix bugs with QS customizer when split shade is active.

* When the QS customizer is active, closing the split shade does not
close the customizer.
* When three-button nav is enabled, pressing back when the QS customizer
is active on the split shade will close the shade, not close the
customizer.

This CL instead adds functions to QS.java so we can check whether the
customizer is active (and close it), so we can handle these cases
appropriately.

Test: Manually verified.
Bug: 201388768
Change-Id: I94208d1cd966975f39328529b9d866fbf27927ac