Replace @hide APIs

NSD associated files are going to move into connectivity
mainline module in T, so need convert all hidden APIs to formal
API surfaces.

- Replace Slog with Log
- Remove useless implements Watchdog.Monitor
- Replace Build.IS_DEBUGGABLE with Build.isDebuggable()
- Replace Preconditions.checkState90 by checking null directly.
- Replace Lists.newArrayList() by new ArrayList<>()
- Replace DumpUtils.checkDumpPermission() by checking DUMP
  permission directly.

Bug: 206702844
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: I6bc478b852c10591959d9e0615af63e675532abe