Bugfix for leaking BroadcastReceiver

Came across this while running ```SimpleSync```:

```
E/ActivityThread( 4490): Activity com.salesforce.androidsdk.ui.sfhybrid.SalesforceDroidGapActivity has leaked IntentReceiver com.salesforce.androidsdk.smartsync.phonegap.SmartSyncPlugin$SyncReceiver@b1759d18 that was originally registered here. Are you missing a call to unregisterReceiver()?
E/ActivityThread( 4490): android.app.IntentReceiverLeaked: Activity com.salesforce.androidsdk.ui.sfhybrid.SalesforceDroidGapActivity has leaked IntentReceiver com.salesforce.androidsdk.smartsync.phonegap.SmartSyncPlugin$SyncReceiver@b1759d18 that was originally registered here. Are you missing a call to unregisterReceiver()?
E/ActivityThread( 4490): 	at android.app.LoadedApk$ReceiverDispatcher.<init>(LoadedApk.java:805)
E/ActivityThread( 4490): 	at android.app.LoadedApk.getReceiverDispatcher(LoadedApk.java:606)
E/ActivityThread( 4490): 	at android.app.ContextImpl.registerReceiverInternal(ContextImpl.java:1430)
E/ActivityThread( 4490): 	at android.app.ContextImpl.registerReceiver(ContextImpl.java:1410)
E/ActivityThread( 4490): 	at android.app.ContextImpl.registerReceiver(ContextImpl.java:1404)
E/ActivityThread( 4490): 	at android.content.ContextWrapper.registerReceiver(ContextWrapper.java:467)
E/ActivityThread( 4490): 	at com.salesforce.androidsdk.smartsync.phonegap.SmartSyncPlugin.initialize(SmartSyncPlugin.java:101)
E/ActivityThread( 4490): 	at org.apache.cordova.PluginEntry.createPlugin(PluginEntry.java:96)
E/ActivityThread( 4490): 	at org.apache.cordova.PluginManager.getPlugin(PluginManager.java:281)
E/ActivityThread( 4490): 	at org.apache.cordova.PluginManager.execHelper(PluginManager.java:232)
E/ActivityThread( 4490): 	at org.apache.cordova.PluginManager.exec(PluginManager.java:227)
E/ActivityThread( 4490): 	at org.apache.cordova.ExposedJsApi.exec(ExposedJsApi.java:53)
E/ActivityThread( 4490): 	at com.android.org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
E/ActivityThread( 4490): 	at com.android.org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:27)
E/ActivityThread( 4490): 	at android.os.Handler.dispatchMessage(Handler.java:102)
E/ActivityThread( 4490): 	at android.os.Looper.loop(Looper.java:136)
E/ActivityThread( 4490): 	at android.os.HandlerThread.run(HandlerThread.java:61)
D/AndroidRuntime( 4670): 
D/AndroidRuntime( 4670): >>>>>> AndroidRuntime START com.android.internal.os.RuntimeInit <<<<<<
```