Fix *ExtentWriterTest unittest.

These test were creating local files in the same directory where the
test is running from, which doesn't work on Android. This patch
leverages other newer temp file creation utilities that also take
care of removing the temp files.

Bug: 26955860
TEST=/data/nativetest/update_engine_unittests/update_engine_unittests --gtest_filter=*ExtentWriterTest.*

Change-Id: If5cd506ae0a7ca85b6fd5395a0982f00775836c7
diff --git a/common/utils_unittest.cc b/common/utils_unittest.cc
index 24468b9..f840a75 100644
--- a/common/utils_unittest.cc
+++ b/common/utils_unittest.cc
@@ -280,10 +280,10 @@
 void GetFileFormatTester(const string& expected,
                          const vector<uint8_t>& contents) {
   test_utils::ScopedTempFile file;
-  ASSERT_TRUE(utils::WriteFile(file.GetPath().c_str(),
+  ASSERT_TRUE(utils::WriteFile(file.path().c_str(),
                                reinterpret_cast<const char*>(contents.data()),
                                contents.size()));
-  EXPECT_EQ(expected, utils::GetFileFormat(file.GetPath()));
+  EXPECT_EQ(expected, utils::GetFileFormat(file.path()));
 }
 }  // namespace