Return Error-wrapped injection result in dispatcher

When finding input targets, the injection may either fail, or it could
succeed and return valid input targes. To better reflect this contract,
return an Error-wrapped InjectionResult from
findTouchedWindowTargetsLocked.

Bug: 211379801
Flag: EXEMPT refactor
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I5b37b7195639d40d6426c246c7b0fdecc4d27527