Remove NetworkStack dependencies on hidden members

 - DhcpClient only shares its handler with IpClient, and NetworkMonitor
   has its own handler: remove Protocol.BASE_DHCP,
   Protocol.BASE_NETWORK_MONITOR
 - Remove dependency on Network.netid in NetworkMonitor
 - Remove dependency on Sets.newArraySet in DhcpServingParams
 - Remove dependency on formatDuration() in DhcpClient
 - Replace isMetered() with hasCapability() in NetworkMonitor
 - Use WifiManager.isScanAlwaysAvailable instead of reading setting

Test: atest FrameworksNetTests NetworkStackTests
Bug: 112869080
Change-Id: Ieef54d847ddc081fb33cbad0b050b06d2e52548e