[2/n] Avoid flicker to drop a widget that needs a config activity

1. Don't show background in PendingAppWidgetHostView if we are showing preview bitmap
2. Extract bitmap from DropView instead of LauncherAppHostView
3. Letterbox bitmap into PendingAppWidgetHostView's canvas if canvas has different aspect ratio

Fix: 284236964
Flag: aconfig launcher.enable_add_app_widget_via_config_activity_v2 DISABLED
Test: manual
Change-Id: I76de215186b96ffe65c909b28155fb19ac90d4f1