Improve partial connectivity

Improve the design and fix some nits.

Bug: 113450764
Test: 1. Build pass
      2. atest FrameworksNetTests
      3. atest NetworkStackTests
      4. Change captive_portal_https_url to https://invalid.com
      to simulate partial connectivity.
Change-Id: Ia56645841d00d2ed8406cfeacb86a4a27fd58650

• src/com/android/server/NetworkStackService.java[diff]
• src/com/android/server/connectivity/NetworkMonitor.java[diff]
• tests/src/com/android/server/connectivity/NetworkMonitorTest.java[diff]