Apply insets to max bounds instead of parent bounds in letterbox mode.

Applying insets directly to parent bounds don't always work because parent bounds may not be equal to max bounds. For example, this happens when "Developer options > Display Cutout > Hide" option is selected or in One-handed mode.

Fix: 174211584
Test: atest WMShellUnitTests
Change-Id: I7415629aab39e5aac6c038f27494b87667ef84d9