domain: allow reading /proc/sys/vm/overcommit_memory

Needed for jemalloc commit:

2f970c32b527660a33fa513a76d913c812dcf7c
Modify pages_map() to support mapping uncommitted virtual memory.

avc: denied { read } for name="overcommit_memory" dev="proc" ino=10544
scontext=u:r:wificond:s0 tcontext=u:object_r:proc:s0 tclass=file

Bug: 29773242
Change-Id: I78054c1ed576a7998c4ee1d1beca2f610c589c3a