Show letterbox rounded corners behind navbar

Use height of the expanded taskbar to determine if the rounded corners
must appear behind or above the taskbar.

Use mGlobalScale instead of CompatScale for scaling the letterbox,
which fixes rounded corners in case per-app scaling override is
enabled.

Change-Id: I133a6bfc79cdb90f9a04f9c52286cf205dae1fb9
Test: atest WmTests:LetterboxUiControllerTest WmTests:SizeCompatTests
Bug: 262360063