vold: implement resetCheckpoint

It will be used by userspace reboot to reset checkpoint state, to make
sure that when /data is remounted, it will be remounted in checkpointing
if a checkpoint was requested beforee userspace reboot.

Test: /system/bin/vdc startCheckpoint 1
      /system/bin/vdc needsCheckpoint (returns 0)
      /system/bin/vdc resetCheckpoint
      /system/bin/vdc needsCheckpoint (returns 1)

Bug: 135984674
Change-Id: Ia29238686289b4eed93e2fb936a8b3d894b94dc9
diff --git a/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index 03fe258..8e5c53d 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -116,6 +116,7 @@
     boolean supportsCheckpoint();
     boolean supportsBlockCheckpoint();
     boolean supportsFileCheckpoint();
+    void resetCheckpoint();
 
     @utf8InCpp String createStubVolume(@utf8InCpp String sourcePath,
             @utf8InCpp String mountPath, @utf8InCpp String fsType,