aw: Correct doc of addJavascriptInterface()

The Javadoc of addJavascriptInterface() mentioned that the injected
Java object will be injected to main frame, but this is not telling
a full picture. The current implementation will inject the Java
object to all the frames actually.

This CL corrected the misleading wording and add a new warning to
explicitly call it out as a security risk.

Bug: 113336656
Test: make ds-docs
Change-Id: Ia79381d1ab38afa963ea7365526749c14e25238c