Allow system_server to read /proc/vmstat

/proc/vmstat oom_kill counts the number of times __oom_kill_process
was actioned
(https://lore.kernel.org/lkml/149570810989.203600.9492483715840752937.stgit@buzz/)

We want to record this in the context of system_server for tracking
purposes.

Bug: 154233512
Change-Id: I27bcbcd5d839e59a1dca0e87e2f4ae107201654c
Merged-In: I27bcbcd5d839e59a1dca0e87e2f4ae107201654c
Test: build, verify vmstat can be read
2 files changed