LayerStats: Fix a use-after-free

std::string::c_str returns memory that's freed when the string dies. So,
this method should be returning the string, rather than a pointer to
it's freshly-deleted contents.

Caught by our static analyzer.

Bug: None
Test: Builds

Change-Id: I3dce7b60c6cfb4e3bfbd6c0dd6fe30dfe7f4313c
diff --git a/services/surfaceflinger/LayerStats.h b/services/surfaceflinger/LayerStats.h
index 7a190fd..bd17d82 100644
--- a/services/surfaceflinger/LayerStats.h
+++ b/services/surfaceflinger/LayerStats.h
@@ -50,7 +50,7 @@
     // Return the name of the composition type
     static const char* layerCompositionType(int32_t compositionType);
     // Return the name of the pixel format
-    static const char* layerPixelFormat(int32_t pixelFormat);
+    static std::string layerPixelFormat(int32_t pixelFormat);
     // Calculate scale ratios of layer's width/height with rotation information
     static std::string scaleRatioWH(const LayerProtoParser::Layer* layer);
     // Calculate scale ratio from source to destination and convert to string