Fix up PresentationSyncTest

On some devices, the GPU seems to get backed up and starts
spoon-feeding surfaceflinger.  This prevents the drop-old-frame
logic from doing anything, and the (wobbly to begin with) test fails.

Following the proud traditions of my forebears, I have resolved the
problem by inserting a "sleep" call.

Bug 10898363

Change-Id: I3ed62f5a79008f35c65573b616899517a6d52809