Android: Print the error code string from the client.

The update_engine_client needs to translate the numeric ErrorCode to a
string name that can be printed on the output. This patch moves the
ErrorCodeToString() function to a new error_code_utils.{h,cc} pair of
files so it can be included easily from the client binary and uses it
in the Android update_engine_client.

Bug: 25631767
Bug: 25598547
TEST=`update_engine_client --update` prints the error message in a non-Brillo device.

Change-Id: Ib40813924ec676f3e703412de90d389b2596177e
diff --git a/update_engine.gyp b/update_engine.gyp
index 7b0597a..ab20243 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -161,6 +161,7 @@
         'common/clock.cc',
         'common/constants.cc',
         'common/cpu_limiter.cc',
+        'common/error_code_utils.cc',
         'common/hash_calculator.cc',
         'common/http_common.cc',
         'common/http_fetcher.cc',