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/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index 63a30e4..12c00e4 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -110,6 +110,7 @@
     void startCheckpoint(int retry);
     boolean needsCheckpoint();
     boolean needsRollback();
+    boolean isCheckpointing();
     void abortChanges(in @utf8InCpp String device, boolean retry);
     void commitChanges();
     void prepareCheckpoint();