Replace dns_tls with private_dns_mode setting

Also includes:
    - SettingsLib strings used in PrivateDnsModeDialogPreference
      interaction in the Settings app
    - rename ContentResolver "resolver" in methods working with
      DNS resolvers (too confusing)

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net
    - no new failures in SettingsBackupTest nor in SettingsProviderTest
    - manual interaction with developer option works
Bug: 34953048
Bug: 64133961
Change-Id: Ia7502916db9ffa0792e1e500a35e34d06a88e79d

core/java/android/net/ConnectivityManager.java

@@ -619,6 +619,35 @@ public class ConnectivityManager {
      */
     public static final int NETID_UNSET = 0;
 
+    /**
+     * Private DNS Mode values.
+     *
+     * The "private_dns_mode" global setting stores a String value which is
+     * expected to be one of the following.
+     */
+
+    /**
+     * @hide
+     */
+    public static final String PRIVATE_DNS_MODE_OFF = "off";
+    /**
+     * @hide
+     */
+    public static final String PRIVATE_DNS_MODE_OPPORTUNISTIC = "opportunistic";
+    /**
+     * @hide
+     */
+    public static final String PRIVATE_DNS_MODE_PROVIDER_HOSTNAME = "hostname";
+    /**
+     * The default Private DNS mode.
+     *
+     * This may change from release to release or may become dependent upon
+     * the capabilities of the underlying platform.
+     *
+     * @hide
+     */
+    public static final String PRIVATE_DNS_DEFAULT_MODE = PRIVATE_DNS_MODE_OPPORTUNISTIC;
+
     private final IConnectivityManager mService;
     /**
      * A kludge to facilitate static access where a Context pointer isn't available, like in the