Add documentation for new security features in the Android 4.2 release.

Also, fill in some gaps in some gaps in the Android Security overview, such as
support for VPN and cryptography.

Change-Id: I48fe22ffb7ea94b133ba1b0e0a6586e7a5f18938
Merge android-4.2.2_r1 into dev-mips-jb-mr1