Added debugOtaKeyboxFallbackDuration property.

[ Cherry-pick of http://ag/16064434 ]
[ Merge of http://go/wvgerrit/136330 ]

This changes adds a custom debug property for changing the fallback
policy used for the system.  Depending on the value set, the device
will either use a "fast" fallback (30 seconds) or "default" fallback
(~1 day with exponential backoff).  Setting this property to either
"fast" or "default" will end the current fallback if it has been
triggered.

Bug: 187646550
Test: Android unit tests
Change-Id: I5271f96139c1e468242f7fa742668cc791ffcf91

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -360,6 +360,16 @@ class CdmEngine {
   virtual void SetUserId(uint32_t user_id) { user_id_ = user_id; }
   virtual uint32_t GetUserId() const { return user_id_; }
 
+  // Changes the rules used for calculating the fallback duration
+  // when OTA keybox provisioning fails.
+  // Default rules use fallback duration measured in days, with exponential
+  // backoff.
+  // Fast rules use fallback durations of a few seconds, without exponential
+  // backoff.
+  // This method has no effect if OTA keybox is not required.
+  virtual void SetDefaultOtaKeyboxFallbackDurationRules();
+  virtual void SetFastOtaKeyboxFallbackDurationRules();
+
  protected:
   friend class CdmEngineFactory;