Atest: support append multiple serial options

BUG: 209700475

Test: atest-src -v CtsSampleMoblyTestCases -s {device1} -s {device2} -- --replicate-parent-setup --multi-device-count 2
pass --serial {device1} --serial {device2} to TF

Change-Id: I015359738a87d00f7df902e7b1f467b483e44962
3 files changed