Allow skipping timestamp check in certain debug conditions

As proposed in go/ota-downgrade, the android auto team wants to install
a full OTA with the older timestamp. We will only allow it on devices
with userdebug build, and has the property "ro.ota.allow_downgrade"
set. Data wipe is almost inevitable due to the probability of security
patch level and database rollback.

Test: set the property and install a full payload with older timestamp.
Change-Id: I897bbb19bfec820340f791abf3c6d8138995fa90
diff --git a/common/fake_hardware.h b/common/fake_hardware.h
index 3e5a66e..8da5326 100644
--- a/common/fake_hardware.h
+++ b/common/fake_hardware.h
@@ -128,6 +128,8 @@
 
   int64_t GetBuildTimestamp() const override { return build_timestamp_; }
 
+  bool AllowDowngrade() const override { return false; }
+
   bool GetFirstActiveOmahaPingSent() const override {
     return first_active_omaha_ping_sent_;
   }