Fix NPE when adding WebView to ResourcesManager

Framework resources have their mResDir set to null, since the framework
resources are implicitly always included. Guard against null
when checking to see if the Resources mResDir matches the asset path
WebView wants to inject itself into.

Bug: 36953234
Test: manual
Change-Id: Ie3dc0cc1240441a2466585224cdc7c15555c66bf