Set the color space as nullptr on native bitmaps when sRGB is requested

This regression is a fallout from the recent API council feedback
fallout. The default color space used to be always set to null but
it's now the named SRGB instance. This change passes null to native
when the specified color space is known to be sRGB.

Bug: 37496760
Test: runtest managed-provisioning
Change-Id: Ie933c84e429a682a58ee253b57b77bd61b88ee5e