EmulatorPkg/MpService: modify AP state should hold a mutex

Add mutex when change AP state.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15668 6f19259b-4bc3-4df7-8a09-765794883524