graphics: improve preloading for IMapper default impl

When GraphicBufferMapper::preloadHal dlopens the default impl, make sure
the gralloc module is also dlopen'ed.

Bug: 269664560
Test: android.view.cts.PixelCopyTest#testVideoProducer on grunt

Change-Id: I57418338d5212fb9a2c1ea82eba63d855f69ff32
diff --git a/graphics/mapper/2.1/default/passthrough.cpp b/graphics/mapper/2.1/default/passthrough.cpp
index c7f0cf5..c99c984 100644
--- a/graphics/mapper/2.1/default/passthrough.cpp
+++ b/graphics/mapper/2.1/default/passthrough.cpp
@@ -19,6 +19,14 @@
 using android::hardware::graphics::mapper::V2_1::IMapper;
 using android::hardware::graphics::mapper::V2_1::passthrough::GrallocLoader;
 
+// Preload the gralloc module such that GraphicBufferMapper::preloadHal is
+// meaningful
+class GrallocPreloader {
+public:
+    GrallocPreloader() { GrallocLoader::loadModule(); }
+};
+static GrallocPreloader sGrallocPreloader;
+
 extern "C" IMapper* HIDL_FETCH_IMapper(const char* /*name*/) {
     return GrallocLoader::load();
 }