docs: Edits to the L Preview API overview.

Also added a screenshot of the new "Battery Historian" tool and
screenshots showing managed vs unmanaged apps in the "Launcher"
screen (all screenshots in regular and "2x" size).

This version of the doc is staged to:

http://asolovay.mtv:9111/preview/api-overview.html

Change-Id: I09d2b11a01edacef2800f80ab93823f1a2c44478