Allow system_server to collect app heapdumps (debug builds only)

On debuggable builds, system_server can request app heap dumps
by running something similar to the following commands:

  % adb shell am set-watch-heap com.android.systemui 1048576
  % adb shell dumpsys procstats --start-testing

which will dump the app's heap to /data/system/heapdump. See
framework/base commit b9a5e4ad30c9add140fd13491419ae66e947809d.

Allow this behavior.

Addresses the following denial:

  avc: denied { write } for path="/data/system/heapdump/javaheap.bin" dev="dm-0" ino=150747 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:system_data_file:s0 tclass=file permissive=0

Bug: 20073185
Change-Id: I4b925033a5456867caf2697de6c2d683d0743540
diff --git a/file_contexts b/file_contexts
index b66c2e0..fcb22e9 100644
--- a/file_contexts
+++ b/file_contexts
@@ -243,6 +243,7 @@
 /data/misc/wifi/hostapd(/.*)?   u:object_r:wpa_socket:s0
 /data/misc/zoneinfo(/.*)?       u:object_r:zoneinfo_data_file:s0
 /data/misc/vold(/.*)?           u:object_r:vold_data_file:s0
+/data/system/heapdump(/.*)?     u:object_r:heapdump_data_file:s0
 
 # Bootchart data
 /data/bootchart(/.*)?		u:object_r:bootchart_data_file:s0