Fix non-resizeable app letterboxing when entering desktop windowing

`topActivityLetterboxAppHeight` and `topActivityLetterboxAppWidth` are
now only populated if the application is letterboxed. These values were
previously used for both letterboxed apps and fullscreen nonresizeable
apps as they contained accurate starting bounds for when the app would
enter SCM.

Introduce `topActivityAppBounds` to replace the above values which will
always be populated regardless of the letterbox state.

Flag: com.android.window.flags.enable_windowing_dynamic_initial_bounds
Fixes: 380254503
Fixes: 379741474
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Change-Id: I61dd34648333b2ee92520d181bfb6c060409705c