[Android] Add MODIFY_AUDIO_SETTINGS permission in Android manifest

The volume hardware button can't control music volume when using webaudio
playing music. This is because app has no pemission to set audio volume.

MODIFY_AUDIO_SETTINGS allows application to be able to configure audio settings.
In android audio manager, a function SetAudioMode is added to set audio mode.

BUG=https://crosswalk-project.org/jira/browse/XWALK-622
Merge pull request #1266 from yinli/webaudio_volume

Add MODIFY_AUDIO_SETTINGS permission in Android manifest