Add an IPC for requesting network scores.

Migrating the NetworkScoreManager.requestScores() method to use an
IPC rather than a broadcast.

Defined the new IPC in the AIDL files for NetworkScoreService and
NetworkRecommendationProvider. Added a no-op implementation in
NetworkScoreService and a full implementation in
NetworkRecommendationProvider (plus tests).

Test: adb shell am instrument -e class android.net.NetworkRecommendationProviderTest -w com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
BUG: 33593157
Change-Id: I951f6186aa11ddbe5a94e374173a1fd40b2a4cce