Fix NetworkStackCoverageTests on Q

Fix regressions in the coverage tests when run on a Q device:
- Do not attempt to verify CaptivePortalData in
  IpClientIntegrationTest on Q, since it cannot be set considering that
  the class does not exist on Q.
- Replace HexEncoding.encodeToString(byte[], boolean) with
  HexEncoding.encodeToString(byte[]) in StructNdOptPref64Test.
- Skip ModuleNetworkStackClientTest using DevSdkIgnoreRule instead of
  assumeTrue() in setUp, as the latter would still run tearDown().
- Ignore NetworkStatsUtilsTest: this is a unit test for a test utility,
  and that utility uses R APIs (so can only be used in R-only tests).
- Lazy-initialize LinkProperties with capport attributes in
  NetworkStackNotifierTest as initializing at class creation would
  crash.
- Manually create a INetworkMonitorCallbacks mock in
  NetworkMonitorTest, create capport LinkProperties on demand, and skip
  capport tests that cannot run on Q.

Test: atest NetworkStackCoverageTests
Bug: 152723363
Change-Id: I3ae98e24a542c6313e06bcc787ad5a0a2a1db723