Simplify ThemePicker dependencies

Replace ThemePicker_common_defaults, ThemePicker_srcs and
ThemePicker_res with a single ThemePickerLib android_library.
This will compile ThemePicker_srcs and ThemePicker_res a single
time instead of in every module.

Also replace the ThemePicker_src_overrides with a ThemePickerOverridesLib
java_library.

Bug: 319160342
Test: builds
Test: atest --host ThemePickerRoboTests
Flag: NA
Change-Id: Ia55daf02a86c331d77d33187b1173e13422f3fc1