Fix DWB banner not showing up for split tasks

- Remove DWB banner and thumbnailView from TaskView when being recycled to avoid multiple banners from staying
- Extracted DWB toastg splitbounds update to not be called whenever task icon is loaded
- Refactor DWB toast to remove unused variable, and extract mSplitBannerConfig to be a getter

Fix: 343212887
Test: OverviewImageTest.digitalWellbeingToast
Flag: EXEMPT bugfix
Change-Id: I2dbdc483a7b99de009c9e2a946e1b026c5e4817f