Don't include "error_code.h" in edify/expr.h.

Use forward declartion to avoid pull in the module that contains
error_code.h (trying to move it into libotautil). Otherwise all the
modules that include "edify/expr.h" need to depend on the module that
exports error_code.h.

.cpp sources should include "error_code.h" explicitly to use the enums.

Test: lunch aosp_{angler,bullhead,dragon,fugu,sailfish}-userdebug;
      mmma bootable/recovery
Change-Id: Ic82db2746c7deb866e8cdfb3c57e0b1ecc71c4dc

error_code.h

@@ -17,7 +17,7 @@
 #ifndef _ERROR_CODE_H_
 #define _ERROR_CODE_H_
 
-enum ErrorCode {
+enum ErrorCode : int {
   kNoError = -1,
   kLowBattery = 20,
   kZipVerificationFailure,
@@ -30,7 +30,7 @@ enum ErrorCode {
   kUpdateBinaryCommandFailure,
 };
 
-enum CauseCode {
+enum CauseCode : int {
   kNoCause = -1,
   kArgsParsingFailure = 100,
   kStashCreationFailure,
@@ -51,7 +51,7 @@ enum CauseCode {
   kVendorFailure = 200
 };
 
-enum UncryptErrorCode {
+enum UncryptErrorCode : int {
   kUncryptNoError = -1,
   kUncryptErrorPlaceholder = 50,
   kUncryptTimeoutError = 100,