Move dumpsys to handler thread

I0afdda023208c3f8620cb5b89add66448af596d7 added some assertions
to ensure NetworkAgentInfo can only be accessed from the handler
thread. This is good to eliminate any potential concurrent
accessing problem. Howver, dumpsys runs on binder thread, this
change breaks dumpsys functionality.

Thus, this change:
  1. Copy Handler#runWithScissors() and related class
     locally since they are @hide methods.
  2. Run doDump on the handler thread and waits for return.

Test: adb shell dumpsys connectivity
Test: atest FrameworksNetTests:android.net.connectivity.com.android.server.HandlerUtilsTest
Fix: 303348620
Change-Id: Ic3a22f5d370bbf8c18b1972e86d695b6ad61bd0d