Add stubs for UpdateEngine.AllocateSpaceForPayload

This API preallocates space for a Virtual A/B update.
Right now, it also returns an error (which becomes a
ServiceSpecificException in Java) when space is insufficient. This will
be fixed in a follow up CL.

Test: pass
Bug: 138808058

Change-Id: I587615ba765acb5a52c1918d6a4acc57a95d75f7
diff --git a/binder_bindings/android/os/IUpdateEngine.aidl b/binder_bindings/android/os/IUpdateEngine.aidl
index 1305079..8a5ec71 100644
--- a/binder_bindings/android/os/IUpdateEngine.aidl
+++ b/binder_bindings/android/os/IUpdateEngine.aidl
@@ -45,4 +45,17 @@
   void resetStatus();
   /** @hide */
   boolean verifyPayloadApplicable(in String metadataFilename);
+  /**
+   * Allocate space on userdata partition.
+   *
+   * @return 0 indicates allocation is successful.
+   *   Non-zero indicates space is insufficient. The returned value is the
+   *   total required space (in bytes) on userdata partition.
+   *
+   * @throws ServiceSpecificException for other errors.
+   *
+   * @hide
+   */
+  long allocateSpaceForPayload(in String metadataFilename,
+                               in String[] headerKeyValuePairs);
 }