Update AnimatedImageDrawable docs

Bug: 73529437
Test: No change in behavior, no new tests

Document that it can be inflated from xml, and reference the attributes
that it supports.

Add a comment describing what the class does with respect to animation.

Update the comment for start() to better reflect when the callback is
called.

Change-Id: I660cbc4e185dca8bc7a43a185f91ae0485ef8753