Add rollback-prevention time methods to ref

Merge from master branch of Widevine repo of http://go/wvgerrit/66077
Merge from oemcrypto-v15 branch of Widevine repo of http://go/wvgerrit/64562

Bug: b/78357351

b/62058202 addressed issues with the Level 3 OEMCrypto in guarding
against rollback. This change does something similar for the ref, so
that OEMCrypto vendors have rollback-prevention code they can refer to.

Test: linux/ce cdm unit tests
Test: tested as part of http://go/ag/5501993

Change-Id: I76128c5def2615ecbdbe94e3af1fec4a025be8c1

libwvdrmengine/oemcrypto/ref/src/oemcrypto_usage_table_ref.h

@@ -129,6 +129,8 @@ class UsageTable {
   std::vector<int64_t> generation_numbers_;
   std::vector<SessionContext*> sessions_;
   OldUsageTable* old_table_;
+
+  friend class UsageTableEntry;
 };
 
 }  // namespace wvoec_ref