[Sb] SingleBindableIcon implementation

Small subclass of ModernStatusBarView that provides a default binding so
we can keep the mechanics of the old status bar layout logic away from
newer icon bindings.

The usecase for this class is basically: I want to show an icon that can
potentially update based on the state it gets from its view model.

Follow-up CL has the only current client of this. Note that this is not
flag-controlled because there is no code that uses it yet.

Test: SingleBindableStatusBarIconViewTest
Bug: 311417356
Flag: NONE
Change-Id: I60e55097d47fd5dc8961687ae29abbeec181a144