Fix pip enter/expand from split-screen secondary.

Split up validateWindowingMode into a validity check and
the one that "resolves" split-screen. This way, calling
setWindowingMode on an ActivityStack won't assign a
random mode (it won't convert undefined into split-secondary).
Without this, the task itself was getting set to secondary
(when it should just be inheriting).

Then, piptaskorganizer will reparent to secondary if in
split-mode (thus using the same logic it used to get the
final animation bounds).

Upon enter, instead of changing root-task mode to pinned,
only the leaf task changes (this is because pinned is now
an alwaysCreateStack activity type). WM, unfortunately,
has to reparent to displayarea here so that it doesn't
mess-up the topActivityType reported via onTaskInfoChanged.

Also made pip params come directly from the top activity
rather than be stored on the task. Otherwise, nested
pip activities wouldn't properly report their params
because they would have been set on the root rather
than their direct parent which meant that once the
direct parent became root, it wouldn't have the params

Bug: 154515783
Bug: 144711770
Test: go to split with a pippable in secondary. go to home
      in secondary.
Change-Id: Ibfc1e7f6805bf2641cff676232c1040e2fa025f1