feat(android): change getDocuments return value

[m1ga]

The current getDocuments call will just return a list of the data values but not the ID of the actual document.

[
{  image: '',    name: 'Name1'  },
{  name: 'Nam2'  },
{  name: 'Name3'  }
]

This PR adds the ID as _id:

 [
  { name: 'Name1', image: '', _id: '8zNNaaaCXyeHY5ETaaa' },
  { name: 'Name2', _id: 'NR6NSOOYaaaH4aaaaL' },
  { name: 'Name3', _id: 'SbQoWJ4h8aaauM1aaadS' }
 ]

I've used _id so it won't clash if you defined an id node yourself. I have a previous commit where I've change the structure to id: "...", data: {} but that would break existing apps.

firebase.firestore-android-2.0.0.zip