surfaceflinger: fix screenshot dataspace under WCG

Commit a252d895 (Use GraphicBuffer instead of GBP for screenshots)
removed the ability to query the dataspace of the screenshots.  WM
assumes they are sRGB anyway.  Let's force screenshots to be in
sRGB.

Bug: 79547689
Test: no color shift during Photos rotation
Change-Id: If57cba1b5a50c4e1f9b305af39d9588b3dd1775d