[AOD RONs] Make conversation icon margins symmetrical

The default conversation icon has only left, top, right margins to leave space for the actions container, but we don't use actions in AOD RONs anyway so we can make the margins symmetrical to improve the alignment.

Fix: 402149684
Test: manual
Flag: android.app.ui_rich_ongoing
Change-Id: Ia6362d54a0669e0dbae2d8b43d3624d6acfaa7e1