Return InputTargets instead of TouchedWindows

Before this CL, TouchedWindows were getting returned from
findTouchedWindowTargetsLocked, just to be converted immediately to
InputTargets.

However, TouchedWindows are actually an implementation detail of
TouchState, and should not leak out from there.

In this CL, the first step is taken to fix this abstraction by returning
InputTargets where TouchedWindows are getting returned.

This will allow:
1. Removal of goto from dispatcher
2. Diff-based approach for generating InputTargets from old and new
   touch state

These updates will be attempted in follow-up CLs.

Bug: 211379801
Test: m inputflinger_tests && adb sync data && adb shell -t /data/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: I8b93719994ed383ad0f9bb848d84805470d95db9