Add user affiliation APIs

Make setAffiliationIds public so that it can be used for COMP.
That way we can allow network logging and other features to
work on devices that have a DO and a managed profile.
Those features are currently restricted to single user devices but we'll
open them up to devices where all users are affiliated.

Also create a getter for that API.

Bug: 32326223
Test: m FrameworksServicesTests &&
 adb install \
   -r ${ANDROID_PRODUCT_OUT}/data/app/FrameworksServicesTests/FrameworksServicesTests.apk &&
 adb shell am instrument -e class com.android.server.devicepolicy.DevicePolicyManagerTest \
   -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: Ie443be887a6ca61a7f7a07e137757dceab7eb3d3