Make Drawable#hasFocusStateSpecified public

Bug: 154628299
Test: ColorStateListDrawableTest, DefaultFocusHighlightTest

The system needs this to show keyboard focus. Apps may want to do
something for keyboard focus, and they may also want to write custom
drawables that handle keyboard focus. Making this API public allows them
to do this.

Change-Id: Id6b2df3a14f15db5b7f8ca664362be0b78c3e2c4