Add a network scorer label metadata field to NetworkScoreAppData

This field allows Network Score Services to pass an alternate label for
the scorer.

Bug: 35848510
Test: runtest --path
frameworks/base/services/tests/servicestests/src/com/android/server/NetworkScorerAppManagerTest.java
runtest --path
frameworks/base/services/tests/servicestests/src/com/android/server/NetworkScoreServiceTest.java

Change-Id: Ic28671c1663bd08b2406045d20c150a209d56054