Fix iOS id

iOS must use weighted predicates or it'll return the wrong result when multiple
elements share the same id.

For example:
id('sign_in_label').click currently matches a static text and a button. The
static text does nothing, so we'd prefer to return an element that has
interactive abilities such as the button.

If someone wanted to match the static text using an id anyway, they could do
s_text resolve_id 'sign_in_label'
Merge pull request #1116 from bootstraponline/ios_id

Fix iOS id