Implement legacy_prop_name

Getter will fall back to legacy_prop_name if prop_name doesn't exist or
empty. This is to help owners deprecate old properties and move to new
properties.

Bug: 152824209
Test: create a cc_binary and test on a device manually
Change-Id: I34d8e7e46043ec0da5e8911a9c8d74ad7bae29ce
diff --git a/sysprop.proto b/sysprop.proto
index 4bb850a..4833603 100644
--- a/sysprop.proto
+++ b/sysprop.proto
@@ -61,6 +61,7 @@
   string enum_values = 6;
   bool integer_as_bool = 7;
   bool deprecated = 8;
+  string legacy_prop_name = 9;
 }
 
 message Properties {