Adjust splash screen icon sizes for Wear

This change moves some constant factors which are used
to determine sizing/scaling behaviour on splash screens
from inline declarations to overlayable resources that
can be customized to suit respective form factors.

Bug: b/278274396
Bug: b/277059561
Test: manual launch of splash screen apps

Change-Id: I8f04e4007d94dbd9113b3f0fdd7036bfb51bbd9b