overlay: writeback: Add dumpsys info

Add dumpsys information from writeback data structure.
Remove some unused fields, formatting to save space.

Change-Id: I808ce67aae16e27c3aa6d6db45983929b1386b69
diff --git a/liboverlay/overlayWriteback.cpp b/liboverlay/overlayWriteback.cpp
index 5c6d773..e7d25b0 100644
--- a/liboverlay/overlayWriteback.cpp
+++ b/liboverlay/overlayWriteback.cpp
@@ -215,6 +215,13 @@
     return mOpFmt;
 }
 
+void Writeback::getDump(char *buf, size_t len) const {
+    utils::getDump(buf, len, "WBData", mFbData);
+    char str[4] = {'\0'};
+    snprintf(str, 4, "\n");
+    strncat(buf, str, strlen(str));
+}
+
 //static
 
 Writeback *Writeback::getInstance() {