[HWUI] Fix invisible views are still shown in popup window

Symptom: Invisible views are still shown in popup window

Root Cause: glClear isn't called if there is no draw command

Solution: Even if there is no draw command, it still calls glClear
          by startFrame().

Reproduce steps: App set one view in a popup window to invisible,
                 it will be still visible.

Change-Id: I36bc7b752434ad388fbb80ea63341ac778f48d35