Document usesCleartextTraffic app-level attribute.

This updates the documentation of the application element of
AndroidManifest.xml to mention the new android:usesCleartextTraffic
attribute.

This also updates the Javadoc of
android.security.NetworkSecurityPolicy and ApplicationInfo to match.

Bug: 19215516
Change-Id: I5c221b56addae8988b4db51994d18ae379d0fbe3