Define a meta-data attribute for a carrier icon.

This is just a centralized key that a carrier app and various eSIM
infrastructure can agree upon. It may be used to allow the carrier app
to show a custom icon instead of its launcher icon when some UI is
rendering the identity of the carrier. Use of this meta-data is
entirely up to callers.

Fixes: 62580015
Test: TreeHugger
Change-Id: Iaaab8cb8b5108d92b8f687d213daa7917f53fe65