[Notif redesign] Change color of conversation icon bg

We were using the same circle drawable for the conversation icon as we
do for the small icon, in order to crop conversation icons to a circle.

However, for the small icon we generally apply a color to this drawable,
so in that case the default color (a dark grey) didn't really matter.
In the case of conversations though, we can have conversation icons with
a transparent background, which lead to the dark grey circle showing
behind. This looked okay in dark mode, but not in light mode, so this
change introduces a new background drawable with a semi-transparent
color that looks good in any theme.

Fix: 406224300
Test: manual, receive a Gmail space notification and check that it looks good in both normal conversations and minimal HUNs, with dark mode on and off
Flag: android.app.notifications_redesign_templates
Change-Id: I7caf1c930c708fc048175a2864ece1665f81475a