Add BUILD_USERNAME and BUILD_HOSTNAME

These replace $USER and `hostname`, which will soon become stable values
due to sandboxing the build on Linux.

Bug: 122270019
Test: check build.prop
Change-Id: I7493e45a2a2defbdba5d82596cb91d68480f0187
diff --git a/core/main.mk b/core/main.mk
index 9fd1c35..4f81c21 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -80,7 +80,7 @@
 ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
   # BUILD_NUMBER has a timestamp in it, which means that
   # it will change every time.  Pick a stable value.
-  FILE_NAME_TAG := eng.$(USER)
+  FILE_NAME_TAG := eng.$(BUILD_USERNAME)
 else
   FILE_NAME_TAG := $(file <$(BUILD_NUMBER_FILE))
 endif