Controls - Structure persistence

Allow structure to be persisted and restored. Assume structure can be
null in XML, to ease migration. Null values from Control will be
mapped to an empty string in ControlInfo. Reduce binding controller
to a single binding at a time.

Bug: 148207527
Test: atest ControlsFavoritePersistenceWrapperTest
ControlsControllerImplTest ControlsBindingControllerImplTest

Change-Id: I220212666258311bd58b498d4d579977c11d3fae