Selectively expand notifications inside bundles

- Single notifications are fully expanded
- Groups show 5 children when collapsed instead of 2 (now with a flag check!)

Test: RowAppearanceCoordinatorTest
Fixes: 431264935
Flag: com.android.systemui.notification_bundle_ui
Change-Id: Ie12d3fe6dc84373a9c6116904ef76554b7378c4b