Eliminate CAP_SYS_MODULE from system_server

Right now, the system_server has the CAP_SYS_MODULE capability.  This allows the
system server to install kernel modules.  Effectively, system_server is one
kernel module load away from full root access.

Most devices don't need this capability. Remove this capability from
the core SELinux policy. For devices which require this capability,
they can add it to their device-specific SELinux policy without making
any framework code changes.

In particular, most Nexus devices ship with monolithic kernels, so this
capability isn't needed on those devices.

Bug: 7118228
Change-Id: I7f96cc61da8b2476f45ba9570762145778d68cb3