fix(interpolate): do not create directives for constant media URL attributes

By creating attribute directives that watch the value of
media url attributes (e.g. `img[src]`) we caused a conflict
when both `src` and `data-src` were appearing on the
same element. As each directive was trying to write to the
attributes on the element, where AngularJS treats `src` and
`data-src` as synonymous.

This commit ensures that we do not create create such directives
when the media url attribute is a constant (no interpolation).

Because of this (and because we no longer sanitize URLs in the
`$attr.$set()` method, this commit also updates `ngHref` and
`ngSrc` to do a preliminary sanitization of URLs in case there
is no interpolation in the attribute value.

Fixes #16734