SerializeFlags is a namespace.

The SerializeFlags becomes a namespace that contains
the original class definition under type SerializeFlags::Type
and static constants.

Consequently, all SeralizeFlags methods can
now be constexpr.

Also added static tests.

Test: libvintf_test
Test: vintf_object_test
Bug: 110711640
Change-Id: I786a1f77e19defc2ab2d025de3ed7cfe48f4fbce
diff --git a/AssembleVintf.cpp b/AssembleVintf.cpp
index 050059b..faab3e1 100644
--- a/AssembleVintf.cpp
+++ b/AssembleVintf.cpp
@@ -702,7 +702,7 @@
     Istream mCheckFile;
     bool mOutputMatrix = false;
     bool mHasSetHalsOnlyFlag = false;
-    SerializeFlags mSerializeFlags = SerializeFlags::EVERYTHING;
+    SerializeFlags::Type mSerializeFlags = SerializeFlags::EVERYTHING;
     std::map<KernelVersion, std::vector<NamedIstream>> mKernels;
     std::map<std::string, std::string> mFakeEnv;
 };