If media overlay is playing, audio doesn't stop when app is backgrounded or device is turned off

[msintov]

There is a bug where media overlay audio doesn't stop playing when the app is backgrounded or the device is turned off.

Repro steps:

1. Launch SDKLaunder-Android
2. Open a book with a media overlay
3. Play the media overlay
4. Background the app or touch the power button to turn off the display

Actual results: audio continues playing
Expected results: audio stops

One potential fix is to call ReadiumSDK.reader.toggleMediaOverlay() if the media overlay is playing in WebViewActivity.onPause(). Seems to work.

[danielweck]

The proposed solution sounds good. Please note that the Readium native application "launchers" are minimalistic by design, so there may be a number of other user experience / device integration issues.

[msintov]

Finally created a pull request for this here: #74