Fixes theming setting to add only necessary properties

`android.theme.customization.system_palette` is being set with both
`system_palette` and `accent_color`. But these properties are only
necessary when `color_source` is set to `preset`. Fixed with this
change.

Bug: 404539643
Test: Manual
Flag: com.android.systemui.hardware_color_styles
Change-Id: I3d9014a57b94a20191300668ef38db7da32882bb