gralloc4-vts: fix GetLargeReservedRegion test

The existing test never assigns a handle to bufferHandle so
mGralloc->getReservedRegion() will always be called with
nullptr and will always return with BAD_BUFFER.

Bug: b/146515640
Test: VtsHalGraphicsMapperV4_0Target
Change-Id: I85dac94956c6b9db5b4a19faeb16fc947f7457e1
diff --git a/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp b/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
index 6cc5e34..a783eaa 100644
--- a/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
+++ b/graphics/mapper/4.0/vts/functional/VtsHalGraphicsMapperV4_0TargetTest.cpp
@@ -2124,8 +2124,14 @@
 
         Error err;
         mGralloc->getAllocator()->allocate(
-                descriptor, 1,
-                [&](const auto& tmpError, const auto&, const auto&) { err = tmpError; });
+                descriptor, 1, [&](const auto& tmpError, const auto&, const auto& tmpBuffers) {
+                    err = tmpError;
+                    if (err == Error::NONE) {
+                        ASSERT_EQ(1, tmpBuffers.size());
+                        ASSERT_NO_FATAL_FAILURE(bufferHandle =
+                                                        mGralloc->importBuffer(tmpBuffers[0]));
+                    }
+                });
         if (err == Error::UNSUPPORTED) {
             continue;
         }