Snapshot Task with the app window crop for Recents

Snapshot a specific task and its children instead of taking a snapshot
of the whole screen with max and min layer values. This makes it clear
what the desired screenshot is instead of trying to determine what the
max and min z layers should be.

In the process of updating the snapshot code, I split the code that
captures a layer from the code that captures the whole screen. This
simplified the code so the capture screen doesn't need to invoke the
frame calculations since it will be capturing the whole screen anyway.

Test: Recents snapshots work correctly without the IME, status, and
navigation bar

Change-Id: I8776c1ddb9cd8a23a482b045720960702796fd5f