[20/n Dispatcher refactor] Allow access to all TouchedWindowHandles

In this CL we introduce following methods to allow iterating over all
touchedWindowHandles.
1. forAllTouchedWindows
2. forAllTouchedWindowsOnDisplay

Bug: 367661487
Bug: 245989146
Test: atest inputflinger_tests
Flag: EXEMPT refactor
Change-Id: I65b2b48d73e11c068d208339931b919e419e2893