surfaceflinger: Description init style change

We move the initialization of fields to Description's
declaration, and switch to using the "default" constructor and
destructor.

This is basically a style change, should essentially have no
difference in behavior, although it does subtly change
Description into being a POD (which might be useful for future
work, but should have no current impact).

Test: Built aosp_arm-eng
Change-Id: I79f91f422ff6ad55dbd9507ee6655f30377339a8