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