Improve startBackNavigation stability

  - Use the focused window instead of the topApp window
    - Instead we now rely on WindowManagerService to get
     the focused window.
    - SystemUI does not have ActivityRecord so we can't rely
     on the top window of the Task to find the correct
      window on which the callback will be called.

  - Introduce a Builder for BackNavigationInfo
    - This reduces the number of variable needed outside the synchonized
    block.
    - It also reduces the number of early return of BackNavigationInfo
    instances

  - Adding log messages to help further debug the method.

Test: BackNavigationControllerTests
Test: Manual dismiss of SystemUi dialog in QS
Bug: 216604581
Fixes: 221458292
Change-Id: I9ba2c7f89956f34d6338824502c210b3e58dc076

Introduce builder for BackNavigationInfo

Change-Id: I14b4a4b3abc8f417998b7b32831cb3d5c4faa491