Set CDMA roaming/subscription mode throws ISE if not CDMA

Since the phone can switch to/from CDMA, returning when not CDMA could
cause issues later on. This essentially reverts ag/13862397 and updates
the documentation to call out more clearly when ISE is thrown.

Test: make
Fix: 175665470
Change-Id: Ie6ab0fd4b317d66a74a7c667a0ed6a54ee56c150