[Android] Add loadAssetFileAndWaitForTitle and apply in XWalkCoreTest cases

Instrumentation tests may fail because the titles may be verified
before they are changed. This commit introduced a new interface named
"loadAssetFileAndWaitForTitle" to wait until the title be changed.

BUG=#984 (Partially)
Merge pull request #1037 from DonnaWuDongxia/waitForTitle

[Android] Add loadAssetFileAndWaitForTitle and apply in XWalkCoreTest cases