Enhance toString on ConferenceParticipant.

In preparation for logging this state more often, add pii obscuring to
critical fields, and add string representation of state.

Bug: 30086285
Change-Id: I6c06f8d46f1524ed7f1e4d01ad37ab67b6d12ed4

telecomm/java/android/telecom/ConferenceParticipant.java

@@ -114,13 +114,13 @@ public class ConferenceParticipant implements Parcelable {
     public String toString() {
         StringBuilder sb = new StringBuilder();
         sb.append("[ConferenceParticipant Handle: ");
-        sb.append(mHandle);
+        sb.append(Log.pii(mHandle));
         sb.append(" DisplayName: ");
-        sb.append(mDisplayName);
+        sb.append(Log.pii(mDisplayName));
         sb.append(" Endpoint: ");
-        sb.append(mEndpoint);
+        sb.append(Log.pii(mEndpoint));
         sb.append(" State: ");
-        sb.append(mState);
+        sb.append(Connection.stateToString(mState));
         sb.append("]");
         return sb.toString();
     }