Permit dropping caches from the shell through sys.drop_caches.

*   Permits setting the sys.drop_caches property from shell.
*   Permits init to read and write to the drop_caches file.
*   Can only be set to 3 (drop_caches) and 0 (unset).

Bug: 178647679
Test: flashed user build and set property; no avc denials.
Test: flashed userdebug build and dropped caches w/o root.
Change-Id: Idcedf83f14f6299fab383f042829d8d548fb4f5d
diff --git a/private/property_contexts b/private/property_contexts
index a3cd9e0..e637665 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -260,6 +260,10 @@
 # Property that is set once ueventd finishes cold boot.
 ro.cold_boot_done       u:object_r:cold_boot_done_prop:s0
 
+# Properties that control performance operations.
+# Leave space to later set drop_caches to 1, 2, and 4.
+perf.drop_caches        u:object_r:perf_drop_caches_prop:s0 exact enum 0 3
+
 # Charger properties
 ro.charger.                 u:object_r:charger_prop:s0
 sys.boot_from_charger_mode  u:object_r:charger_status_prop:s0 exact int