docs: Changed "Runtime Permissions" doc to "Permissions"

Per message from Joe, changed name of feature from "Runtime
Permissions" to just "Permissions". Still sometimes referred to it as
"runtime permissions" in descriptive text, as appropriate.

I considered renaming the feature page to ./permissions.jd , but
given the short time, I decided against it--there were too many
ways a file rename could break things, and this is just a temporary
"preview" page anyway.

See first comment for stage location.

Change-Id: I0ce36d99ea2377cf9fa7ac72b2f182dedb5c38e8