Add GetActiveBootSlot API to boot control interface

This can be used to verify OtaStatus. If on disk pref indicates that we
switched slot and system did not reboot, check with boot control.

Test: th
Change-Id: I416d81a3d68d8c6a204a2c28df6c567ff18d0e49
diff --git a/common/boot_control_interface.h b/common/boot_control_interface.h
index 321174e..2de21a1 100644
--- a/common/boot_control_interface.h
+++ b/common/boot_control_interface.h
@@ -93,6 +93,11 @@
   // bootloader will attempt to load the |slot| marked as active. Note that this
   // method doesn't change the value of GetCurrentSlot() on the current boot.
   virtual bool SetActiveBootSlot(Slot slot) = 0;
+  // Get the active slot. In other words, the slot which will be used on
+  // next system reboot. This should match the |slot| parameter of last
+  // successful call to |SetActiveBootSlot|.
+  // Return 0xFFFFFFFF if underlying HAL doesn't support this operation.
+  virtual Slot GetActiveBootSlot() = 0;
 
   // Mark the current slot as successfully booted asynchronously. No other slot
   // flags are modified. Returns false if it was not able to schedule the