[Android] Support keeping-screen-on for web apps

Provide '--keep-screen-on' arguments in packaging tool
and apply keep-screen-on flag to the activity,
then the activity will request keep screen on for the whole life.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1411
Merge pull request #1825 from jinbang/keeping_screen_on

[Android] Support keeping-screen-on for web apps