Fix ThemePicker for use_resource_processor: true

Give ThemePicker resources their own package (com.android.themepicker),
and reference resources with the package name specified in the manifest
of the library that contains the resources.

Bug: 319160342
Test: m javac-check
Test: atest --host ThemePickerRoboTests
Flag: NA
Change-Id: Ifbe99d75fedb3959b976d5d8f66778ae5f37e466