Allow switching keyboard layout to null.

Previously, PersistentDataStore#switchKeyboardLayout doesn't allow
switching keyboard layout to null, which means default layout.
As a result, when no keyboard layout is corresponding to the new
subtype, previous keyboard layout continues to be used on switching
subtpyes.

Bug: 27750850
Change-Id: Ie5c6ef7944ade48907a5b92986578f9f40b0f499