Set iconsSizePx of Taskbar's DeviceProfile directly

- After ag/20691430, iconScale no longer scale app icons (I should rename it later, after QPR2), so we'll need to set the desired iconSize directly for Taskbar's DeviceProfile
- Extracted setting iconSizePx and testSizePx out of updateIconSize, as they are no longer affected by scaling. This allows TaskbarActivityContext to set desired iconsize and then call updateIconSize

Bug: 256976071
Test: Test Taskbar icon size in different density
Change-Id: I741c178b59e0d8ecb4c63689e68767622329379c