API review: access field by method

- InvalidPacketException, public field should be a method so
  add getter to get error code.
- KeepalivePacketData, public fields should be methods so
   add getter for fields.

Bug: 151322799
Test: atest FrameworksNetTests
      atest FrameworksWifiTests
      atest FrameworksTelephonyTests: some failure in CarrierAppUtilsTest
Change-Id: Id01e6135193716cc21bba11da529bf1507a954f7