Add hidden constants for communicating about local network agents

framework/src/android/net/NetworkCapabilities.java

framework/src/android/net/NetworkRequest.java

framework/src/android/net/NetworkScore.java

...src/com/android/server/connectivity/NetworkAgentInfo.java

...ils/devicetests/com/android/testutils/DevSdkIgnoreRule.kt

tests/common/java/android/net/NetworkCapabilitiesTest.java

tests/cts/net/src/android/net/cts/NetworkRequestTest.java

...android/server/connectivityservice/CSKeepConnectedTest.kt