Add reason to focus change event log

We want to start changing focus in ID for multiple
reasons and this change pulls out the focus change
logic from setInputWindows. In addition, it adds a
reason to the focus event logs for better tracking.

Test: adb logcat -b events and check focus change reasons
Test: atest FlickerTests, inputflinger_tests
Bug: 151179149

Change-Id: I12cb2f02dfbdd7376fe3699a32c4e8d4d03ba57d