kmsaudiomixer: Use liveadder instead of an adder

live adder mixes audio according the their timestamps and waits for some
milli-seconds before trying doing the mixing. Because this element
does not propagate EOS signal on its src pad, we have also removed
probe, so we will lose data in buffer are queued in live adder element

Change-Id: I94e7e86193d195d18f9e7d966240e178c66c855c