Move ACTION_CLEAR_DNS_CACHE to ConnectivityManager

- As API review feedback, move ACTION_CLEAR_DNS_CACHE form Intent
  to ConnectivityManager.

- Rename to "android.net.action.CLEAR_DNS_CACHE" because of lint
  suggestion.
frameworks/base/packages/Connectivity/framework/src/android/net/
ConnectivityManager.java:449: error: Inconsistent action value;
expected `android.net.action.CLEAR_DNS_CACHE`, was
`android.intent.action.CLEAR_DNS_CACHE` [ActionValue]

Bug: 183937999
Test: atest FrameworksNetTests
Test: atest ActivityTaskManagerServiceTests
Test: atest android.permission2.cts.PermissionPolicyTest
Test: atest CtsNetTestCases
Change-Id: Iae8aa0ba10dfc7581f0cfaab82643edbee789e2f