AAPT2: Add support for clearer in-progress public attributes

Before, the ID assigned to a public resource without an explicitly set id
was more difficult to figure out. It would be the next available ID.

AAPT2 introduces a new way to specify public attributes in progress.

<public-group type="attr" first-id="0x0101047f">
  <public name="foo" />
  <public name="bar" />
  ...
</public-group>

The IDs assigned to each resource is auto-incremented starting from `first-id`.
This also keeps resource's with the same type grouped together so that
the auto-incrementing nature is evident.

Also, due to how AAPT2 was implemented, this is required :P

Change-Id: I95ea92ad0405e87ed0b1766879bb2f1d9d0b636e