[25/n] Reduce LetterboxUiController-Reachability dependencies

Removes code from LetterboxUiController about double tap handling.

Move LetterboxUiController#getLetterboxReasonString() to CompatUtil
class with related tests.

Flag: EXEMPT refactor
Bug: 336807329
Test: atest WmTests:LetterboxUiControllerTest
Test: atest WmTests:SizeCompatTests
Test: atest WmTests:AppCompatUtilsTest

Change-Id: I76b7363988541c77ef6d52904e5dc827a1105234