Fix AppCache when either no path or an invalid path is supplied

The bulk of the change is in to external/webkit:
https://android-git.corp.google.com/g/155448

This part of the change simply adds some optimizations to avoid syncing to
native when not required, and cleans up the JavaDoc.

Bug: 5745181
Change-Id: I0009c1868755fe211ec2dc31bd007d393aabfbe3