[1/2][PiP2] Fix enter-pip for AE case

Make sure DefaultMixedHandler properly animates
enter-pip case in PiP2.

1. Detect that the PiP activity is in a TF which isn't a root task
in ATMS, setting FLAG_IN_TASK_WITH_EMBEDDED_ACTIVITY in the request.

2. Resolve this request in DMH, augmenting thru the PiP handler to
get the right startWCT for us to actually move to a pinned task;
this should be a no-op for PiP1.

3. In DefaultMixedTransition, when animating for AE case, resolve
and separate out both the PiP Task and the PiP config-at-end activity
case. Then route to a more generic mPipHandler#startAnimation()
if both changes exist.

This should fix the animation for Activity Embedding + enter PiP flow,
but there is still an issue that the non-PiP activity is also marked as
config-at-end - hence, the client doesn't draw till the enter is over;
the latter issue still causes entireScreenCovered() failures.
This will be addresssed separately.

Bug: 380025892
Flag: com.android.wm.shell.enable_pip2
Test: atest SecondaryActivityEnterPipTest
Change-Id: Iee8cbc2d86f9c7a463223e44899261f333467d7d