Add a dedicated method to check task windowing mode

Trying to get a task information via ActivityManager#getAppTasks()
will not report tasks that aren't associated with calling package
by the root activity package name. For activities that are from
a different package the task wasn't found, so the windowing mode
couldn't be checked and WM Extensions didn't report any display
features.

This CL adds a dedicated method to check the task windowing mode
for a given activity token.

Bug: 236295340
Test: ActivityEmbeddingIntegrationTests#testDisplayFeaturesWithEmbedding_differentPackage
Change-Id: If2dbc337c4b8cb909914cc28ae4db28a82ff9de3