[usecase-embedding][XWALK-2327] Add one testpoint for show and hide in embedding sample.

- Add one testpoint in embedding sample for onshow and onhide.
- Remove the unserviceable code because XWalkView has done these.

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 0, block 0