fix($rootScope): when adding/removing watchers during $digest

Previously, adding a watcher during a `$digest` (i.e. from within a watcher),
would result in the next watcher getting skipped. Similarly, removing a watcher
during a `$digest` could result in the current watcher being run twice (if the
removed watcher had not run yet in the current `$digest`).

This commit fixes both cases by keeping track of the current watcher index
during a digest and properly updating it when adding/removing watchers.

Fixes #15422

Closes #15424