Never show the large icon in MessagingStyle

Also documented this behavior, and improved the consistency of the
Style javadocs a little bit.

Fix: 403535357
Test: manually verified the large icon is not shown, but is still being
used as a fallback when there's no shortcut icon
Flag: EXEMPT BUGFIX

Change-Id: Ib6f3c435a3ea257724160e52f0beca1f90897dce