Enhance error handling logic to restore split-screen

Unusual use cases result in the main split empty. This CL uses focus
listener to improve error handling logic which restores split screen
from unexpected poor status.

Bug: 235193477
Test: atest MultiWindowTests
      atest WMShellUnitTests
Change-Id: I636c33821699c9774f687a89fc31b8de2859df7e
4 files changed