Add a function pointer to get active boot slot

bootcontrol 1.2 adds a new function to get the active boot slot
on the next boot. Since the getter is closely related to the gpt
implementation of bootcontrol, we add the header here and expose
to clients of libhardware. Devices with an old HAL should never
call this new getter function.

Bug: 173808057
Test: Vts tests for bootcontrol

Change-Id: I19135ed16bf0ba546e0a8d9cb2d521ec0a9c5bbc

include/hardware/boot_control.h

@@ -125,7 +125,14 @@ typedef struct boot_control_module {
      */
     int (*isSlotMarkedSuccessful)(struct boot_control_module *module, unsigned slot);
 
-    void* reserved[31];
+    /**
+     * Returns the active slot to boot into on the next boot. If
+     * setActiveBootSlot() has been called, the getter function should return
+     * the same slot as the one provided in the last setActiveBootSlot() call.
+     */
+    unsigned (*getActiveBootSlot)(struct boot_control_module *module);
+
+    void* reserved[30];
 } boot_control_module_t;