SurfaceTextureClient: Add ISurfaceTexture getter.

This change adds a getter method to SurfaceTextureClient to get the
ISurfaceTexture object with which the SurfaceTextureClient is
communicating.

Bug: 4086509
Change-Id: Ifec621e0fe5392a5be56b2348fdc54067cbebcdd
diff --git a/libs/gui/tests/SurfaceTextureClient_test.cpp b/libs/gui/tests/SurfaceTextureClient_test.cpp
index 359b71c..25109f6 100644
--- a/libs/gui/tests/SurfaceTextureClient_test.cpp
+++ b/libs/gui/tests/SurfaceTextureClient_test.cpp
@@ -35,6 +35,11 @@
     sp<SurfaceTextureClient> mSTC;
 };
 
+TEST_F(SurfaceTextureClientTest, GetISurfaceTextureIsNotNull) {
+    sp<ISurfaceTexture> ist(mSTC->getISurfaceTexture());
+    ASSERT_TRUE(ist != NULL);
+}
+
 TEST_F(SurfaceTextureClientTest, QueuesToWindowCompositorIsFalse) {
     sp<ANativeWindow> anw(mSTC);
     int result = -123;