Add tombstone_config_prop and move related prop

tombstoned.max_tombstone_coun becomes tombstone_config_prop to remove
exported*_default_prop

Bug: 155844385
Test: tombstoned is running and logcat shows no denials
Change-Id: I57bebb5766d790dc52d40a6d106f480e0e34fa4e
diff --git a/private/property_contexts b/private/property_contexts
index d56c89c..9eb8251 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -484,7 +484,7 @@
 sys.usb.ffs.ready     u:object_r:ffs_control_prop:s0 exact bool
 sys.usb.ffs.mtp.ready u:object_r:ffs_control_prop:s0 exact bool
 
-tombstoned.max_tombstone_count u:object_r:exported3_default_prop:s0 exact int
+tombstoned.max_tombstone_count u:object_r:tombstone_config_prop:s0 exact int
 
 vold.post_fs_data_done u:object_r:vold_config_prop:s0 exact int