Use material3's dynamic color schemes in App Header

Uses dynamicLightColorScheme and dynamicDarkColorScheme based on the
app's theme to style the header. This allows the App Header to use
light/dark variants of color tokens even if the app theme and system
theme are not aligned, whereas before, "inverse" tokens (resolved with
the system context) had to be used for apps not following the system
theme, which are much more limited than non-inverse tokens.

Bug: 328668781
Test: App Header of light app on dark system theme and viceversa
always follows the app theme and focus / unfocused headers are
differentiated

Change-Id: I9321726551a54c3ac1b1bf3fe12b2ee29dc7a5be