Request audio focus when playing; Pause audio when audio focus is lost; resume playing when audio focus is granted again.

This closes #88

A local file lacking the file:// or cdvfile:// prefix is not
played because resourceURL isn't validated to later on in the
play chain. avPlayer is initialized during create(), but won't
initialize without the validated resourceURL.. This closes #85

This closes #84

   1- [iOS] Only perform 'seek' operation if both avPlayer and avPlayerItem are ready,
        send error to client code if they aren't ready.
   2- Deplay Media File Playing by a few seconds so that 'seek' operation happens only
        after enough buffering has been done.

CB-10476 - fix problem where callbacks were not invoked on android due to messageChannel being overridden by callbackContext in every execut() call. This closes #78

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X-ref: https://github.com/apache/cordova-plugin-device/pull/48

This will still intermittently fail test 19 but this is more to do with
the success criteria of that rest.  rate is applied and works.

This reverts commit e7f9f048.

Fixed example referencing non-existent variable media, fixed to use my_media variable.