sf: vds: Fixes and code cleanup for VirtualDisplaySurface

1. Remove unused variable declarations

   Remove static variable sForceHwcCopy that is not required since moving
   to an implementation that uses a local variable mForceHwcCopy.

2. Add a condition to reduce log spam from advanceFrame hook

   When mForceHwcCopy is true, we override the composition type to MIXED.
   Therefore, we need to check whether we are in this scenario and add
   checks to satisfy the state machine requirements and reduce log spam.
   In particular, by setting mForceHwcCopy we can now expect to get an
   advanceFrame when composition type is MIXED and our previous state was
   PREPARED or GLES_DONE.

Change-Id: I1f47d6d6f01deec17da9069dea32c382c8f39b02