Only show people icons for Notifications that are MessagingStyle

Instead of only checking that the channel is used for important conversations. Note that this will also prevent the shortcut or large icon from being displayed in that case, but that seems to be the intent of the code (naming like isImportantConversation, showPeopleAvatar, etc).

Also several non-logic changes, especially in the test file (formatting, fixing inverted assertEquals, named arguments, etc).

Fixes: 295816841
Test: Unit test
Change-Id: I7e0bced9207576889f9c092ee41158ac8cf78246