Update docs to reflect adoptable behavior.

Update storage related docs on Context to be consistent, and to call
out relevant Environment methods.  Start calling it "shared" storage,
and only mention external for historical reasons.  Mention that there
isn't much benefit to using emulated storage over private data
directories to help guide developers to safer locations.

Point out which paths can change over time, so developers know to
only persist relative paths.

Update Environment docs to reflect how they behave for the new
class of adopted storage devices.

Bug: 24251945
Change-Id: Ie5ab337649b4740dfd7594997bbb19c4969cfd2f
(cherry picked from commit 59d28dc8)