Fix colors in importance and conversation priority preferences

This would have been simpler and faster, but the ChannelPanelActivity
was giving me a context where Material colors always inflated in light
theme even if the device was in dark theme, so I had to work around it.

On the plus side, whoever next needs to change these colors will find
them neatly organized in themes.xml!

Bug: 284380876
Test: manual
Change-Id: I2eee877fe44d307a96d60168d0317444ff002669