Bundle header tweaks

* fix bundle expander color to match other expanders
* fix bundle icon background to match expander
* remove the sometimes-transparent background of preview icons
* reduce the stroke width to account for the built-in padding of app icons

Fixes: 423946203
Test: manual & screenshots
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I2740fdc6e644e37b11980d778a9c55403cd7c47d