Add feature flag for STK: SEND USSD and SET UP CALL commands

This commit adds a feature flag for the new feature:
STK - SEND USSD and SET UP CALL

Bug: 404099417
Test: build
Flag: com.android.internal.telephony.flags.support_stk_command_ussd_and_call
Change-Id: Iebb8f3f49c014399c353aec75fff7682bc4ff06a