Fix dump of active config

Currently x-dpi and y-dpi appear as invalid characters in
dumpsys. This CL formats them as strings.

Test: adb shell dump SurfaceFlinger
Change-Id: I0332cc4634db5594e0cd406fe656e5eb9a96a144
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 240db35..a3e1a17 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -4824,9 +4824,10 @@
         const auto activeConfig = getHwComposer().getActiveConfig(*displayId);
         std::string fps, xDpi, yDpi;
         if (activeConfig) {
-            fps = std::to_string(1e9 / getHwComposer().getDisplayVsyncPeriod(*displayId)) + " fps";
-            xDpi = activeConfig->getDpiX();
-            yDpi = activeConfig->getDpiY();
+            fps = base::StringPrintf("%.2f Hz",
+                                     1e9f / getHwComposer().getDisplayVsyncPeriod(*displayId));
+            xDpi = base::StringPrintf("%.2f", activeConfig->getDpiX());
+            yDpi = base::StringPrintf("%.2f", activeConfig->getDpiY());
         } else {
             fps = "unknown";
             xDpi = "unknown";