Make bitmap nullable in ViewCapture

Fixes local compilation in SysUI studio.
The callback interface has @NullableDecl
annotation that Kotlin compiler treats as
Kotlin nullable.
Not sure why it compiles successfully in
soong though (different compiler version?).

Bug: 236188940
Change-Id: Ia8cf1dd25c3ea6a162ad3dddd58b4bf4c0f889a9
Test: build from sysui studio

packages/SystemUI/screenshot/src/com/android/systemui/testing/screenshot/ViewCapture.kt

@@ -93,8 +93,8 @@ fun View.toBitmap(window: Window? = null): Bitmap {
             Futures.addCallback(
                 captureToBitmap(window),
                 object : FutureCallback<Bitmap> {
-                    override fun onSuccess(result: Bitmap) {
-                        continuation.resumeWith(Result.success(result))
+                    override fun onSuccess(result: Bitmap?) {
+                        continuation.resumeWith(Result.success(result!!))
                     }
 
                     override fun onFailure(t: Throwable) {