Change taskAffinity for DesktopWallpaperActivity

Given that we want DesktopWallpaperActivity to always stay in the background and not interrupt other Activities, a different Task makes it stay on its own. This also fix the recording bug, where we were starting MediaProjectionAppSelectorActivity in the same task.

Fixes: 436478247
Test: m && manual test
Test: StartAppScreenRecordingFromNotificationTest
Flag: EXEMPT small bugfix; change in AndroidManifest
Change-Id: Ic809a5199a3e24e01e38ec000039d17b2c27aa02