[Android] Fix that instrumentation test is not using ShellActivity

The correct usage of instrumentation test is to direct operate
the Activity in target package instead of creating one.

Change the test util base Class to get XWalkRuntimeClient from
XWalkRuntimeActivityBase.

TODO(hengzhi): Fix the same issue with CoreShellTest and RuntimeShellTest
Merge pull request #1323 from wang16/test_refine

[Android] Fix that instrumentation test is not using ShellActivity