cherrypick from lmp-docs docs: Listening for restrictions-changed

Original Change-Id: I19c7150a9f36de226d00ad6d8db0e9210d5d135d

Per bug 19848206, it turns out that APPLICATION_RESTRICTIONS_CHANGED
is only sent to dynamically-declared listeners, *not* to listeners
declared in the manifest. Updated doc to note this. (The code snippet
already showed how to dynamically declare a listener.)

See first comment for doc stage location.

bug: 19848206
Change-Id: I6dc5f39dc6eed71cf2e303564ea906be3832d8a4