Display black screen when backing to app on Android

[ininallen]

Summary: Display black screen when backing to app on Android

BUG DETAILED DESCRIPTIONS
Install XWalkRuntimeLib.apk & XWalkAppTemplate.apk on your Android device, launcher XWalkAppTemplate in app Launcher, Press Back key to exit app after XWalkAppTemplate launched,launch XWalkAppTemplate again,the screen went black when launching xwalk

Environment:
Android: build 60

EXACT STEPS LEADING TO PROBLEM:

1. Install XWalkRuntimeLib.apk & XWalkAppTemplate.apk on your Android device
2. Launcher XWalkAppTemplate in app Launcher
3. Press Back key to exit app after the XWalkAppTemplate launched
4. Launch XWalkAppTemplate again and observe the screen
5. The screen went black when back

EXPECTED OUTCOME:
Screen display normally, no black

ACTUAL OUTCOME:
Screen went black when back

USER IMPACT:
It impact user experience

[sqliu]

merge this to #430 , the same issue and I already have a fix and wait for merge.

[ininallen]

Is this a duplicated issue with #430