Temporarily stop proxying to cmd

Unless the APK is streamed, proxying pm install commands to the cmd command
will generally not work on any system that has SElinux enforcement. This
is due to the system sever [where cmd commands are run] not having access
to the directories where files are uploaded to. We'll re-enable the proxy
once we determine an appropriate, safe location for uploaded APKs.

Bug: 25769800
Change-Id: I73b2da0b03fe8bd9ce29e20b37cb4eeff9b491b4