SystemUI: Don't forcefully convert wallpaper to SRGB

The issue occurs when the user tries to set as wallpaper an image that
is not SRGB.
Fix the issue by making sure the greyscale bitmap has the same
color space as the original image ( which would default to SRGB otherwise
and in some cases the R+G+B would overflow the 255 limit )

Test: https://drive.google.com/file/d/11rxzYvPcIOh_8GvS4XSC3YtbW3CecE-O/view

Change-Id: I2b0c40eef608f423dbbee4a80df55c271f9856c3