Cleanup networkstack-client

 - Use framework-annotations-lib instead of framework-annotations, to
   avoid including annotation .class files in the output .jar
 - Remove dependency on net-utils-framework-common, by moving
   KeepalivePacketDataUtil.fromStableParcelable to its only user,
   TcpKeepaliveController.
 - Build against module_current to allow using
   framework-annotations-lib without conflicts with annotation classes
   in the system_current stubs. Have NetworkStack stubs link against
   just the AIDL libraries (not networkstack-client), as the system_29
   stubs cannot use a module_current library.

Test: atest NetworkStackTests ConnectivityCoverageTests
Change-Id: Ic07b7bdd1ef57d690644ce09d9d5a7851ef9763d