Bring the group tag back to VectorDrawable.

This is the first step, including:
1. Bring the group tag back by reverting.
2. If there is no group tag, then treat all the paths as a single group.
Make sure our test cases covering both cases.

At the same time, fix one tiny UI bug in the test.

TODO:
1. Move the rotation to the group tag.
2. Add translation/scale to the group tag.
3. Convert the group into a tree hierarchy.

This reverts commit 404211630ba3bb865109434aba4433e7541b39d3.

Conflicts:
	graphics/java/android/graphics/drawable/VectorDrawable.java

Change-Id: Ife65bb6a7fc85f9947124b8e356cd5b82119796a