[Android] Fix the build issue caused by different Android OS Version.

Since different Android APIs are imported in different Android OS version,
the OS version with lower API level uses the API in high API level, which
causes issue. This fix resolves the issue by adding the control condition
in the API uses of different API levels.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1081
Merge pull request #1657 from ShDavidliu/APILevel

[Android] Fix the build issue caused by different Android OS Version.
[Android] Split crosswalk into core layer and runtime layer.

Originally the two layers are compiled into one Java package. It needs to
split them into two different directories for building because core will
be published as an Android library without runtime layer code.
Besides that, there are also 2 major changes:
1. Move Java extension mechanism and extensions inside core layer.
2. Change the 'runtimelib' to 'runtime_lib'.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1041