Scope locks for mirrorDisplay

Prevents deadlock between threads grabbing mStateLock and
mCreatedLayersLock by reducing the scope of mStateLock in
mirrorDisplay so that only one lock is held at a time

Fixes: b/411570724, b/415746305
Test: presubmit, atest SurfaceFlinger_test
Flag: EXEMPT, bug fix
Change-Id: Icdfedaf847d4720d67bb6e5138cdf6ba34001b37