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/VoldNativeService.h b/VoldNativeService.h
index 12a93f5..d02b262 100644
--- a/VoldNativeService.h
+++ b/VoldNativeService.h
@@ -162,6 +162,8 @@
     binder::Status bindMount(const std::string& sourceDir, const std::string& targetDir) override;
 
     binder::Status destroyDsuMetadataKey(const std::string& dsuSlot) override;
+
+    binder::Status getStorageSize(int64_t* storageSize) override;
 };
 
 }  // namespace vold