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/hardware_interface.h b/common/hardware_interface.h
index 0140588..4a64c3e 100644
--- a/common/hardware_interface.h
+++ b/common/hardware_interface.h
@@ -122,6 +122,10 @@
   // Returns the timestamp of the current OS build.
   virtual int64_t GetBuildTimestamp() const = 0;
 
+  // Returns true if the current OS build allows installing the payload with an
+  // older timestamp.
+  virtual bool AllowDowngrade() const = 0;
+
   // Returns whether the first active ping was sent to Omaha at some point, and
   // that the value is persisted across recovery (and powerwash) once set with
   // |SetFirstActiveOmahaPingSent()|.