Added a preference link to manage external sources in the app info
screen just like other special access permissions.
Test: Manually checked that the link appears on apps once their app op
is set to allow or deny.
Also, added some basic robolectric unit tests:
make -j32 RunSettingsRoboTests
Bug: 35481942
Change-Id: If687778b7a64fb7d278508b78d06272253a98e3e
