Handle relaunching pausing desktop tasks

- Detect when we are relauching a task within a desk root and readjust
their surface position which is already relative to their parent,
otherwise we would apply an extra offset.

- When a desk root (or one of its child tasks) is relaunched from
recents, only the selected tile (in exploded view) and the root will
appear as "opening" in the merged transition because the other tasks
in the root are not changing visibility or order and thus are not
participating in the transition for the launch of the other sibling.

In such cases, the opening task (and root) would've been saved in
|mPausingTasks| and will be moved to |mOpeningTasks| while also
falling into the |onlyOpeningPausedTasks| case. This means this
is considered a |returningToApp| case and the only additional work left
to do is to reorder the "opening" tasks to top (i.e. the desk root and
selected task) after reordering the "pausing" tasks to top (i.e. the
siblings). Note that this is especially important even though the desk
and its selected task were already moved to front because "pausing"
tasks may contain siblings that we don't want to move back on top of
the selected task and may also contain the DesktopWallpaperActivity
task, which should not be on top of the root or any child.

Also notable that falling into |returningToApp| (a desk in this case) is
intentional, as this ensures DesktopTasksController's handling
of #handleFinishRecents keeps the desk in an active state.

- Also applies rounded corners to these opening freeform tasks.

Bug: 420744057
Bug: 413378320
Flag: com.android.window.flags.enable_multiple_desktops_backend
Test: manual - with exploded view enabled swipe of from a desk with 2+
tasks and select them individually - verify exit overview and reentry
into desk is WAI.
Test: manual - from a desk, swipe up to home and verify deactivation is
WAI
Test: atest RecentsTransitionHandlerTest

Change-Id: I0f6ae35ef1c3fcf5be7627cc05604d9055652699