surfaceflinger: Set correct initial value for mActiveColorMode

HWC always initializes with the native mode, setting
mActiveColorMode to sRGB here without actually calling
setColorMode on HWC will cause framework to think that it is in
sRGB mode already, when the mode has not been set yet.

Bug: 38207860
Test: Compile and boot
Change-Id: I492c07ce542f5a448ff4c7d2f18e72629fc6cd78