Fix WebView vulnerability by disallowing file access

Fix webvuew vulnerability inside captiveportal activity by disallowing
access private file in app's sandbox.

Bug: 150610071
Test: Build
Change-Id: I67e695478476b6ee9cf21ed41213f25441d9776a
(cherry picked from commit fef654fffc4783f71f94600597e2fb69249eeb35)
Merged-in: I67e695478476b6ee9cf21ed41213f25441d9776a
1 file changed