Fix plugin.xml -- needs framework tag; requires android 4
Add script to download Crosswalk library
Fix the white flash issue when apps pause and resume

onShow() and onHide() are called by XWalkView internally.
Fix engine declaration to only apply to Android
Update fetch_libs.sh to get Crosswalk 8.37.189.0
Install build.gradle file after downloading libraries
Don't pause timers when app is backgrounded.