Use Offscreen Surface for Split Screen

tools/viewer/sk_app/android/RasterWindowContext_android.cpp

Index: tools/viewer/sk_app/android/RasterWindowContext_android.cpp
diff --git a/tools/viewer/sk_app/android/RasterWindowContext_android.cpp b/tools/viewer/sk_app/android/RasterWindowContext_android.cpp
index a5f9a657feadce25c0f022d69f1738f20c8748fa..e2e2c0b16c228027cb37270059c31c1c6350463d 100644
--- a/tools/viewer/sk_app/android/RasterWindowContext_android.cpp
+++ b/tools/viewer/sk_app/android/RasterWindowContext_android.cpp
@@ -67,7 +67,7 @@ sk_sp<SkSurface> RasterWindowContext_android::getBackbufferSurface() {
         const int bytePerPixel = fBuffer.format == WINDOW_FORMAT_RGB_565 ? 2 : 4;
         SkImageInfo info = SkImageInfo::Make(fWidth, fHeight,
                                              fDisplayParams.fColorType,
-                                             kOpaque_SkAlphaType,
+                                             kPremul_SkAlphaType,
                                              fDisplayParams.fColorSpace);
         fBackbufferSurface = SkSurface::MakeRasterDirect(
                 info, fBuffer.bits, fBuffer.stride * bytePerPixel, nullptr);