Ignore local transforms when mirroring a partial hierarchy

When mirroring a partial hierarchy remove the local transform
on the mirrored root.  This is necessary because we are placing
the mirrored root under a new parent layer and its original
position relative to the old parent is no longer relevant. This
fixes offsets when mirroring a freeform window.

Flag: com.android.graphics.surfaceflinger.flags.detached_mirror
Fixes: 337845753
Test: atest libsurfaceflinger_unittest
Change-Id: I0453026217f62868a4f6d1362cdc8187baf3f9b0