SELinux policy for reading system ion heap size

This grants access for system server to read system ION heap size from
debugfs on all builds.
Experiments on userdebug builds show that large ION heap is correlated
with severe LMKs. This CL will help confirm the finding on bigger
population.

The size of the system ion heap is read by the system server similarly
to other memory measurements.

Example /sys/kernel/debug/ion/heaps/system (repeated lines and columns
removed):
          client              pid             size
----------------------------------------------------
 audio@2.0-servi              765             4096
 audio@2.0-servi              765            61440
 audio@2.0-servi              765             4096
     voip_client               96             8192
     voip_client               96             4096
   system_server             1232         16728064
  surfaceflinger              611         50642944
----------------------------------------------------
orphaned allocations (info is from last known client):
----------------------------------------------------
  total orphaned                0
          total          55193600
   deferred free                0
----------------------------------------------------
0 order 4 highmem pages in uncached pool = 0 total
0 order 4 lowmem pages in uncached pool = 0 total
1251 order 4 lowmem pages in cached pool = 81985536 total
VMID 8: 0 order 4 highmem pages in secure pool = 0 total

Test: m -j
Test: statsd_testdrive returns non-zero value
Bug: 130526489
Bug: 128412961
Bug: 139283562
Change-Id: I5b42a06c3f7fd083150193c6bd291c33e332c23b
(cherry picked from commit a079bde7b91b7e96a78ced2ca07dbeb701f65426)
Merged-In: Icf21f6d502b2d134f28e4fb58f339178b6747dd4

• private/genfs_contexts[Added - diff]
• private/system_server.te[Added - diff]
• public/file.te[Added - diff]
• vendor/qcom/common/hal_dumpstate_impl.te[diff]