AIP review: change to use getter to access a field

Fields in KeepalivePacketData are changed to private and
add getter as API.

Bug: 151322799
Test: atest FrameworksNetTests
      atest FrameworksWifiTests
      atest FrameworksTelephonyTests: some failure in CarrierAppUtilsTest
Change-Id: I6d5be90eafceb2ce0220cd5fc8b86cc20e407999