Generate stable version of NetworkStackStatsLog

Add another genrule to generate stable version of NetworkStatsLog which
compiles against R API. Use this genrule for stable NetworkStack.
The 'current' version of NetworkStackStatsLog contains usage of new SystemApi added in S.

Also use --minApiLevel 29 instead of --supportQ

Bug: 175314831
Bug: 160938178
Test: m statlog-networkstack-java-gen-current
Test: m statlog-networkstack-java-gen-stable
Test: inspect generated stable and current NetworkStackStatsLog files.
Change-Id: I5e1c0e08b1f2ece5cf25ecb7cb919499100dd2a8