[Android] Deprecate xwalk_android_gyp

In M36, android_gyp is still available in upstream, but it is
going to be removed in recent future.
We need to deprecate xwalk_android_gyp together with upstream's
change, and we need to pass '-DOS=android' explicity to it to
generate android building environment.

This change is the prerequisite of the change in crosswalk:
https://github.com/crosswalk-project/crosswalk/pull/2079

BUG=https://crosswalk-project.org/jira/browse/XWALK-1803
Merge pull request #120 from jondong/gyp_xwalk

[Android] Deprecate xwalk_android_gyp
Make replace_version_string global

This is so I can use it in the manifest page to get the current
major version number.
[manifest] Add manifest overview page
[manifest] Add sub-pages for field descriptions
[manifest] Add permissions info and field descriptions
[manifest] Sort sub-pages alphabetically
[manifest] Crosswalk 1 didn't support Android
[manifest] Reposition anchor as it's causing incorrect styling of the heading
[manifest] Start populating sub-pages
[manifest] Flesh out entry points page
[manifest] Update display page
[manifest] Update page about CSP
[manifest] Add sections about special effects to property-specific sections

Setting some properties in the manifest actually modifies how
an application is packaged on Android, rather than affecting the
runtime directly.

Move notes about these effects into the pages specific to the
fields (e.g. permissions, icons) and link to the them from the
"Using the manifest" page.
[manifest] Update icons page
[manifest] Update permissions page
[manifest] Make it clear that some fields have no effect on embedding API

Some fields affect Android packaging, but have no effect on Android
at runtime (i.e. if used in a manifest file which is loaded into
an embedded Crosswalk inside an Android app).
[manifest] Update content security policy page
[manifest] Update network access page
[manifest] Update CSP page