Add an isCheckpointing() binder call for adb remount.

This is needed so "adb remount" can avoid writing to /data during a
checkpoint.

Bug: 157540389
Test: manual test
Change-Id: I33a691da3b99343acfc1e8ddf68a14504c3bfbe1
diff --git a/VoldNativeService.h b/VoldNativeService.h
index 4578dc0..7065b04 100644
--- a/VoldNativeService.h
+++ b/VoldNativeService.h
@@ -133,6 +133,7 @@
     binder::Status startCheckpoint(int32_t retry);
     binder::Status needsCheckpoint(bool* _aidl_return);
     binder::Status needsRollback(bool* _aidl_return);
+    binder::Status isCheckpointing(bool* _aidl_return);
     binder::Status commitChanges();
     binder::Status prepareCheckpoint();
     binder::Status restoreCheckpoint(const std::string& mountPoint);