Add @Override annotation to onTouchEvent

Currently, it is not obvious that onTouchEvent is completely overridden
from View.java without calling back into the super method. Add an
@Override to make this explicit.

Also, make the sendEvent function private because no one is using it
outside of the current file.

Bug: 128553584
Test: presubmit
Change-Id: I72f64c5fab4e8386ba5e58bf1b24a7f953c78548