SF: Allow multiple invalid stack layers through to CE

Currently, all duplicate stack layers are denied passage to the
CompositionEngine. This makes sense when dealing with per-display
on-screen content, but breaks apart if for mirrored and off-screen
content.

Allow multiple invalid (-1) stack layers through, as this state is used
to represent stack layers that can be updated safely by multiple sources
or are associated with one source.

Fixes: 397775142
Test: atest SurfaceFlinger_test, atest MultiDisplayTest
Flag: com.android.graphics.surfaceflinger.flags.reject_dupe_layerstacks
Change-Id: Ic103471a8cc997eb30e7f14d08559e351b01b801