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/hardware_android.h b/hardware_android.h
index 5b3c99d..2a8f669 100644
--- a/hardware_android.h
+++ b/hardware_android.h
@@ -53,6 +53,7 @@
   bool GetNonVolatileDirectory(base::FilePath* path) const override;
   bool GetPowerwashSafeDirectory(base::FilePath* path) const override;
   int64_t GetBuildTimestamp() const override;
+  bool AllowDowngrade() const override;
   bool GetFirstActiveOmahaPingSent() const override;
   bool SetFirstActiveOmahaPingSent() override;