[Status Bar] Append "__external" to external icons.

Bug: 255428281
Bug: 265307726
Test: manual: (1) Be in silent mode. (2) Make a phone call. (3) Mute the
phone call. -> Verify that *both* silent *and* mute icons appear in the
status bar. Verify that they can each be toggled independently without
affecting each other.
Test: atest StatusBarIconControllerImplTest (verified that all added
tests would have failed without the changes in this CL)

Change-Id: I54cebf7b9b96c1b333f1f2a9ca3d427e9e765790