Hold active call when placing 2nd call

When "Add call" is placed from Dialer to choose another sub
for a 2nd call, hold the active call on the existing sub.
The dial out to the modem is sequenced after the HOLD completes.

Bug: 326025258
Test: atest TelephonyConnectionServiceTest
Change-Id: I62c33b24e3230f02fb5310f4cc13b636a89452d8
2 files changed