Add Firestore indexes to local emulator

### [REQUIRED] Environment info

**firebase-tools:**
7.15.0

**Platform:**
Ubuntu 18.04.4
### [REQUIRED] Test case

I'm trying to test [our Firestore indexes](https://tutorbook.app/docs/tutorial-Storage.html), but they seem to always be working (regardless of the indexing settings included in `firestore.indexes.json`). Below is the [test case in question](https://github.com/tutorbookapp/tutorbook/blob/develop/firebase/test/test.js#L382):
```javascript
const FILTERS = {
'grade': ['==', TUTOR.grade],
'gender': ['==', TUTOR.gender],
'location': ['==', TUTOR.location],
'type': ['==', TUTOR.type],
'payments.type': ['==', TUTOR.payments.type],
'config.showProfile': ['==', true],
};
const SORTERS = [
'avgRating',
'numRatings',
];

describe('Tutorbook\'s Database Indexing', () => {
const db = authedApp({
uid: PUPIL.uid,
email: PUPIL.email,
access: PUPIL.access,
});
const filterCombos = combinations(Object.keys(FILTERS));
return Promise.all(filterCombos.map(filters => it('lets users filter ' +
'profiles by ' + filters.join(', '), () => {
var query = db.collection('users')
.where('access', 'array-contains-any', PUPIL.access);
filters.map(filter => query =
query.where(filter, FILTERS[filter][0], FILTERS[filter][1]));
return Promise.all(SORTERS.map(sorter =>
firebase.assertSucceeds(query.orderBy(sorter).get())));
})));
});
```

### [REQUIRED] Steps to reproduce

Just try testing any type of composite or collection group query that would (normally) require an index. It will work on the emulator suite regardless of the existence of the required index (but it won't work in production without the index).

### [REQUIRED] Expected behavior

These tests should fail unless there are Firestore indexes (specified in `firestore.indexes.json`) that support the composite indexes tested.

### [REQUIRED] Actual behavior

All the tests succeed (even though they shouldn't).