Remove @hide symbols from NativeDaemonConnector

- FgThread is an service internal class that NativeDaemonConnector
  can not use it after moving into mainline module. Thus, replace
  it by creating a new thread.

- Remove shutting down check because the original problem owner
  netd isn't used NativeDaemonConnector now.

Bug: 206702844
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: Ib2e227ef1e361f96b397abd7c2a88420ad5a2f22