[wrt] Improve tests use pyunit for wrt-manifest-android-tests

Impacted TCs num(approved): New 0, Update 265, Delete 0
Unit test Platform: Android
Android test result summary: Pass 265, Fail 0, Block 0
Merge pull request #1272 from wanghongjuan/wrt/manifest-improve

[wrt] Improve tests use pyunit for wrt-manifest-android-tests