Fix a naming typo for source_build

When parsing the last_install, the variable name is supposed to be
*source_build* instead of *source_version*.

Bug: 28658632
Change-Id: I1e0ed7150e04885f904b6a3efa18bd5cfe17cc96

core/java/android/os/RecoverySystem.java

@@ -730,7 +730,7 @@ public class RecoverySystem {
 
                 if (line.startsWith("time")) {
                     timeTotal = scaled;
-                } else if (line.startsWith("source_version")) {
+                } else if (line.startsWith("source_build")) {
                     sourceVersion = scaled;
                 } else if (line.startsWith("bytes_written")) {
                     bytesWrittenInMiB = (bytesWrittenInMiB == -1) ? scaled :