[Expressive design] Add default background to App Bar Container

Bug: 432377096
Test: visual
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:1e6d6ce576805fcdacf02c0f7f9bc21ee2226675
Merged-In: I188454a7f4a57a4c24d2025bf17c1ee2f94b25d4
Change-Id: I188454a7f4a57a4c24d2025bf17c1ee2f94b25d4
1 file changed