Add 'preparers' template to cts-automated config.

This allows attaching additional target preparers to
prep the device (eg launch an emulator) before starting cts run.

Bug: 174668429
Test: cts-tradefed --dmgr:max-emulators 15 run cts-automated
    --template:map preparers=emulator/local_headless.xml
    --shard-count 15 --abi x86
Change-Id: I64e6743ab71ba1f0c332a7fb7fb7a830af307494
1 file changed