[39/n] Add runOnFilteredItem to LetterboxUtils

This method allows to use a predicate on the key to iterate on
some elements in a Map.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 382423480
Test: atest WMShellUnitTests:LetterboxUtilsTest

Change-Id: I000de492548095f1622e687d96da6ca3237a8e2a