Add support for binding multiple features per slot (1/2)

1) We previously only allowed for binding one ImsService per
slot. This CL relaxes this restriction for compatibility
purposes for RCS UCE.
2) Also include a slot in bound services for EMERGENCY_MMTEL.

Test: atest FrameworksTelephonyTests; atest CtsTelephonyTestCases
Bug: 111305845
Change-Id: Ie3a8aee50b542963328e3569528311df60ead30b