Add vdc volume getStorageSize

StorageStatsManager.getTotalBytes currently takes the size of /data and
rounds up to known probable sizes to guess the size of internal storage.
This is not always correct.

Instead, find the device /data is on and get the size of that device.
This should give a more accurate answer.

Bug: 295358118
Test: vdc volume getStorageSize returns storage size
Change-Id: I907892041b1ce2cd72092a9877ac34c12bf3f254
diff --git a/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index c798959..c7102e3 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -136,6 +136,8 @@
 
     void destroyDsuMetadataKey(@utf8InCpp String dsuSlot);
 
+    long getStorageSize();
+
     const int FSTRIM_FLAG_DEEP_TRIM = 1;
 
     const int MOUNT_FLAG_PRIMARY = 1;