Set alpha value for newly created dim surface.

Newly created dim surface has alpha set to 1 (opaque),
but it is assumed in dim animation code that it is 0 (transparent).
When new dim surface is created and expected dim value is calculated to 0
then alpha is never set making screen black (dut to default aplha=1)
when dim surface is shown.

services/java/com/android/server/WindowManagerService.java

@@ -10986,6 +10986,7 @@ public class WindowManagerService extends IWindowManager.Stub
                 try {
                     mDimSurface = new Surface(session, 0, -1, 16, 16, PixelFormat.OPAQUE,
                             Surface.FX_SURFACE_DIM);
+                    mDimSurface.setAlpha(0.0f);
                 } catch (Exception e) {
                     Log.e(TAG, "Exception creating Dim surface", e);
                 }