Move unittest constants to header

Allows unittests to easly access these constants w/o having to declare
extern const char* bla;

Test: th
Change-Id: I0bf6637430104c3d621277a725721ca61c6ac544
diff --git a/download_action_android_unittest.cc b/download_action_android_unittest.cc
index fef2d24..0f7ecdc 100644
--- a/download_action_android_unittest.cc
+++ b/download_action_android_unittest.cc
@@ -32,6 +32,7 @@
 #include "update_engine/common/mock_action_processor.h"
 #include "update_engine/common/mock_http_fetcher.h"
 #include "update_engine/common/mock_prefs.h"
+#include "update_engine/common/testing_constants.h"
 #include "update_engine/common/test_utils.h"
 #include "update_engine/common/utils.h"
 #include "update_engine/payload_consumer/install_plan.h"
@@ -46,9 +47,6 @@
 using testing::Return;
 using testing::SetArgPointee;
 
-extern const char* kUnittestPrivateKeyPath;
-extern const char* kUnittestPublicKeyPath;
-
 class DownloadActionTest : public ::testing::Test {
  public:
   static constexpr int64_t METADATA_SIZE = 1024;