Clear Bitmap density to render correctly in Overview

Fix: 419196782
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: Manual - steps in bug
Test: Manual - Media projection (other usage of the screenshots)
Test: Manual reverify of b/421205443
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c435c3e2c6f406a9f80cb8bfd67e93eb9f45cbee)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:304662aa4e29cdae769b416aa1c81f3c67379290)
Merged-In: I6ba77aa7d24f85ab9890d2c900f283369c247a1f
Change-Id: I6ba77aa7d24f85ab9890d2c900f283369c247a1f