[DocsUI M3] Update layout for app bar

* Remove the padding top for main container in compact screen.
* Update the app bar background color when it's pinned.
* Use the same padding start/end for Medium/Expanded screens.

Check the bug for before/after comparison.

Bug: 377771946
Test: Manual inpsection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I2768f963265a2c394e39e5be1bca13440ab81e58