Allow system_server to read all /proc files

system_server scans through /proc to keep track of process
memory and CPU usage. It needs to do this for all processes,
not just appdomain processes, to properly account for CPU and
memory usage.

Allow it.

Addresses the following errors which have been showing up
in logcat:

  W/ProcessCpuTracker(12159): Skipping unknown process pid 1
  W/ProcessCpuTracker(12159): Skipping unknown process pid 2
  W/ProcessCpuTracker(12159): Skipping unknown process pid 3

Bug: 15862412
Change-Id: I0a75314824404e060c6914c06a371f2ff2e80512