Move *_app into their own file

app.te covers a lot of different apps types (platform_app, media_app,
shared_app, release_app, isolated_app, and untrusted_app), all
of which are going to have slightly different security policies.

Separate the different domains from app.te. Over time, these
files are likely to grow substantially, and mixing different domain types
is a recipe for confusion and mistakes.

No functional change.

Change-Id: Ida4e77fadb510f5993eb2d32f2f7649227edff4f