Clean up fuzz helper classes

- Remove OEMCrypto state changes hidden in constructors and destructors.
- Use composition instead of inheritance to structure classes.
- Avoid calling non-trivial destructors for objects with static
  lifetime.

Merged from https://widevine-internal-review.googlesource.com/168497
Merged from https://widevine-internal-review.googlesource.com/171170
Merged from https://widevine-internal-review.googlesource.com/171171
Merged from https://widevine-internal-review.googlesource.com/171870

Change-Id: I20476a7b1132d11f011b8650ec01e3c2dc3fc0e8

libwvdrmengine/oemcrypto/test/fuzz_tests/oemcrypto_generate_signature.cc

@@ -1,13 +1,13 @@
-#include "properties.h"
+#include "oemcrypto_fuzz_helper.h"
 #include "oemcrypto_session_tests_helper.h"
-
-using namespace wvoec;
+#include "properties.h"
 
 static bool is_init = false;
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-  SessionUtil session_helper;
+  wvoec::SessionUtil session_helper;
   if (!is_init) {
+    wvoec::RedirectStdoutToFile();
     wvoec::global_features.Initialize();
     wvoec::global_features.RestrictFilter("*");
     wvutil::Properties::Init();
@@ -18,7 +18,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   OEMCrypto_EnterTestMode();
   session_helper.EnsureTestROT();
 
-  Session s;
+  wvoec::Session s;
   s.open();
   s.GenerateDerivedKeysFromKeybox(session_helper.keybox_);
 
@@ -32,6 +32,5 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
 
   s.close();
   OEMCrypto_Terminate();
-
   return 0;
 }