Add indexOf method to ArrayMap & ArraySet

Getting the indexOf is useful for doing compound operations
like:

int i = set.indexOf(key);
if (i >= 0) {
    Object o = set.valueAt(i);
    o.blah();
    set.removeAt(i);
}

Change-Id: I3d4b77d1461ba969fc6b4d332d52d4d084b5b53c