Update taskbar overflow view to match the spec

This includes:
1) updating the color token used in dark mode,
2) adjusting sizes of individual app icons and their strokes.

See screenshots in http://b/383556682#comment7.

Bug: 383556682, 380386440
Flag: com.android.launcher3.taskbar_overflow
Test: manual
Change-Id: Ia05d803c5602552ce4f3e4675411b281107ac843