docs: Docs for Support Library 24.2.0 release

Includes updates to the features doc, to reflect the v4 support
library's division into several smaller modules. Also modified
the transitions doc to note the support library's support for
transitions in API levels 14-19.

Since we're deprecating support for API levels below 9, I changed
several examples to use higher API levels.

See first comment for doc stage location.

bug: 30505601
Change-Id: Ic06a023f705893c91ff3b86d0af8cfed957eb724