Reenable Strictmode disk reads checks in WebViewFactory.

In an earlier release we had to allow disk reads during a part of
WebView initialization, now that this fault is fixed we can remove that
allowance.

Bug: 63324842
Test: Manual: ensure calling new WebView() doesn't cause StrictMode disk
read violation.
Test: run WebViewHostSideStartupTest.testStrictMode()

Change-Id: If7b4ad790c67469806c255427d86e5fd28d19f35