Add support for automatic focusability determination

New android:focusable attribute value "auto" and corresponding constant
FOCUSABLE_AUTO specifies that the framework can determine a view's
focusability. For now, the only "auto" logic is that focusable = clickable.

This has the effect of making clickable views focusable by default
unless the developer has explicitly set them focusable=false.

Bug: 31963283
Change-Id: Icce768fa701966970ea6b3469b33a4ef97ebc3ea
Test: Added CTS test, related CTS passes, made a test-app which
      enumerates all clickable/focusable combinations to verify