Fix errors being hidden by Protobuf warning-disabling

(This is a merge of http://go/wvgerrit/140850.)

This patch fixes a number of minor issues in the codebase (mostly
instances of 0-as-nullptr, but also some member shadowing and a missing
override) that were being hidden by the fact that depending on Protobuf
disables these diagnostics. And which will be unhidden when a later
patch removes that behavior from Protobuf.

Bug: 208304830
Test: x86-64
Change-Id: I4b0b1264748880b3726a6388d589868d898f949e

libwvdrmengine/cdm/core/test/test_base.cpp

@@ -764,7 +764,7 @@ bool TestLicenseHolder::DeriveKey(const std::vector<uint8_t>& key,
     return false;
   }
 
-  if (!CMAC_Init(cmac_ctx, &key[0], key.size(), cipher, 0)) {
+  if (!CMAC_Init(cmac_ctx, &key[0], key.size(), cipher, nullptr)) {
     LOGE("DeriveKey(): CMAC_Init");
     CMAC_CTX_free(cmac_ctx);
     return false;