[Android] Support multiple tabs on XWalkCoreShell

To support multiple instances of XWalkView, need to have a test tool to test it.
This patch adds one additional tab on XWalkCoreShell which contains another XWalkView instance.

BUG=XWALK-1043
Merge pull request #1683 from kurli/runtime-shell-tab

[Android] Support multiple tabs on XWalkCoreShell