[usecase-webapi] Optimize the case of webapi-usecase-w3c-tests for HrtimeAndroid


- Change button to "Waiting" when play, and disable the button. Change back when play end.
- Put the result info "The time of video playing is" below the play button.

Impacted tests(designed|approved): new 0, update 1, delete 0
Unit test platform: [Android]
Unit test result summary: pass 1, fail 0, block 0