[Android] Launch Screen 1st-step implementation

This is the first step implementation of Launch Screen feature.

The workflow in general as below:
Build script:
1. Check whether 'launch-screen' provided in manifest.json.
2. Check 'launch-screen'->'default'->'image' field, and copy the image to 'res/drawable'
   Rename it as 'launchscreen.*'.
3. Set '@drawable/launchscreen' as window background, it will be displayed immediately
   after user tap the app icon.
Note: The drawable will be customized as the spec in the next implementation steps.

Runtime:
1. Check and get 'launch-screen'->'ready_when' field in manifest.json.
2. Display a dialog as launch screen, and set the background '@drawable/launchscreen'.
3. Hide the dialog depends on 'ready_when' condition:
   1. First rendered frame.
   2. First rendered frame & Page load finished
   3. First rendered frame & js:window.show()

TODO:
Test will be committed in another patch
Next step implementation will support the full spec.

Spec: https://docs.google.com/a/intel.com/document/d/17PuNuHRTQuREUpaCvj-eEx7uYi2avd-VW-oaMXMpvwo/edit?pli=1#
Design Doc: https://docs.google.com/a/intel.com/document/d/1YLajlZC7CkBOtEvzD6p6yeBr0HrPsaIGX8Eqhr3oTF4/edit#heading=h.mfsu3bk9566e
Bug: XWALK-637
Merge pull request #1498 from kurli/launchScreen

[Android] Launch Screen 1st-step implementation