Building Crosswalk: Update link to Chromium's AndroidBuild page.

The Install_prerequisites anchor does not exist anymore, so just point
to the page without any anchors.
Building Crosswalk: Update Android build instructions.

* Recommend using a chromium.gyp_env file just like upstream to set
  variables like target_arch and OS.

* Always recommend setting OS=android. That is not strictly necessary
  until Crosswalk 7, but it has since become mandatory because the
  envsetup.sh script has stopped doing it implicitly.

* Unify the instructions for calling xwalk_gyp_android: if one sets all
  variables in chromium.gyp_env, the same command can be called for all
  currently used Crosswalk versions (5, 6 and 7).
Merge pull request #118 from rakuco/update-android-build-instructions

Building Crosswalk: Update Android build instructions.
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