run-as: use mmap to read package list file

This patch uses mmap() to read /data/system/packages.list

This avoids depending on the size of a fixed static buffer
which may happen to be too short for systems with a lot of
packages installed.

Also avoids calling malloc() which we don't want to trust here
since run-as is a setuid program.

Change-Id: I1d640a08b5d73af2fc80546b01c8d970c7f6b514