Fix google-explicit-constructor warnings in soundpool.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: Id1183b2929242cdf1fd0a1558d0e70194a1de9bb

media/jni/soundpool/SoundPool.h

@@ -37,7 +37,7 @@ class SoundPool;
 // for queued events
 class SoundPoolEvent {
 public:
-    SoundPoolEvent(int msg, int arg1=0, int arg2=0) :
+    explicit SoundPoolEvent(int msg, int arg1=0, int arg2=0) :
         mMsg(msg), mArg1(arg1), mArg2(arg2) {}
     int         mMsg;
     int         mArg1;

media/jni/soundpool/SoundPoolThread.h

@@ -40,7 +40,7 @@ public:
  */
 class SoundPoolThread {
 public:
-    SoundPoolThread(SoundPool* SoundPool);
+    explicit SoundPoolThread(SoundPool* SoundPool);
     ~SoundPoolThread();
     void loadSample(int sampleID);
     void quit();