Fix SCM app letterboxing in desktop mode

Use `topActivityLetterboxAppWidth` and `topActivityLetterboxAppHeight`
to calculate initial bound aspect ratio for all unresizeable apps
entering desktop mode. This ensures the aspect ratio matches the one used
to resolve the size compat configuration as the same bounds are used for
the calculation. This prevents letterboxing in desktop mode due to a
discrepancy in aspect ratio.

Flag: com.android.window.flags.enable_windowing_dynamic_initial_bounds
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Fixes: 367334265
Change-Id: I1e2afcda7f84f232a9f96fffa4b513be512238ca