[AppBarLayout] Add setEnterAnchor/setExitAnchor with indirect child

Nobody has claimed this yet.

**Problem**
- layout_scrollFlags has to be set sequentially consistent among children
If button1..5 is vertically layout and I want to show button2(=enterAlways)
I have to set button2..5, same for exit
- collapse with minHeight works well but it requires additional height calculation for user.
If hierarchy is appbarlayout - constraintlayoutA - button1..5, then
I have to calculate set A's minHeight by summing button2..5

**Suggestion**
- Add setEnterAnchor(View) to AppBarLayout and let appbar calculate height
then when scroll down, appbarlayout scrolls down until anchor enter.
anchor view doesn't have to be a direct child.
anchor view needs to be set once so no more scroll|enterAlways cascades.
- setExpand(boolean) can be superseded. (#1922 won't be necessary then)
setExpand(true) -> setEnterAnchor(appbarlayout)
setExpand(false) -> setEnterAnchor(null)
- setExitAnthor(View) can be also set.
exit-anthor-view should be below enter-anchor-view, clearly.

**Alternative**
- Set tag to the anchor view in xml(or java) and let appbarlayout find it by full traverse.
I don't think using tag is appropriate but layout_flag can't be used for indirect child so I couldn't think other way.

**Compatibility**
- What if old(layout_scrollFlag) and new (anchor) behaviors conflict?

If you guys think this is an acceptable feature, I will happily make PR.

Review AppBarLayout's existing layout_scrollFlags, minHeight, and setExpand behavior first, then examine how indirect descendants are represented in the view hierarchy. Clarify the anchor API and its interaction with existing flags before implementation; done means the requested enter and exit behavior works without requiring sequential flags or manual minHeight calculations.

Written by the indexing model from the issue text.