surfaceflinger: Initialize Description field

We default Description::mIsWideGamut to false in the constructor.
This assures we'll have a consistent value if this field is
referenced prior to setWideGamut() being called.

Test: Built on aosp_arm-eng; a variation of this CL has been in use on an oc-mr1-dev branch with no noticable issues.
Bug: 71642473
Change-Id: Id498b2a87e7eb50f64e0af1533d22c52d9bc0942
Merged-In: Id498b2a87e7eb50f64e0af1533d22c52d9bc0942