Improve Javadoc for AnimatedVectorDrawable

This CL adds a summary of what developers can expect from running
AVD on RenderThread.

Also re-structure the class level Javadoc summary to better organize
the examples.

Test: make offline-sdk-docs

BUG: 30402908
Change-Id: I7111f67db5f2ec352730f633bb34f2e89e1c70e9