[Android] Fix Internal extensions js files are not injected with instrumentation test

The bug description on Jira isn't correct. The root cause is instrument
test loads test page as "Data URI scheme". The body of "Data URI scheme"
should be escaped when data type is "text/html". If skip this step, the
page loading will get failed.

BUG=XWALK-99
Please add comment to explain why need a encode here.