Make sure AppCacheTest casts to long

AppCacheTest was multiplying two integers that were too large for
devices that had more free space than 32-bits could represent.

Hopefully no one releases devices with 1TB soon.

The testAppCacheClear test doesn't work currently because it has an
"imax" attribute that doesn't allow the disk to be filled up entirely.
It only writes a couple hundred MB before it gives up.

Bug: 3216419
Change-Id: Ie371f42211bab65f2c767b36d9837ce76922501e