[Android] Set DIP scale and refine the parameter.

This patch is to set the DIP scale in the construction of XWalkContent
and refine the parameter of onScaleChanged().
[Android] Add test case for OnScaleChanged.

This patch is to add test case for OnScaleChanged().
Merge pull request #2314 from wuhengzhi/onScaleChanged_test

[Android] Set DIP scale and add test case for onScaleChanged.