Delete TouchState::source

It's not clear why we have this behaviour. It's currently untested, and
we don't expect the incoming sources to change, either from the
listener, or from the injection. If the sources change, then it should
be the incoming stream that's responsible for canceling the old gesture,
not the dispatcher.

Bug: 211379801
Test: m inputflinger_tests && $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: I5035a470581af9c161b2c7210e8d65823b719598