Fix issue 2045983 ToneGenerator: fix void statement.

There is a void statement at line 917 of ToneGenerator.cpp: mState == TONE_IDLE;
This problem is harmless as in current code this execution path is never taken; it can only happen if a "new" operator fails in prepareWave() which is a case we usually consider as unlikely in android audio framework.

media/libmedia/ToneGenerator.cpp

@@ -914,7 +914,7 @@ bool ToneGenerator::startTone(int toneType) {
                 }
             }
         } else {
-            mState == TONE_IDLE;
+            mState = TONE_IDLE;
         }
     } else {
         LOGV("Delayed start\n");