hwui: ro.hwui.render_ahead=0 queue on high FPS

ro.hwui.render_ahead was behaving the same as -1 when it was set
to zero. Correct it so that ro.hwui.render_ahead will queue frames
when set to zero on any display rate FPS.

Fixes: 131340980
Test: Test presence or not of timestamps with -1,0,1,2 settings
Change-Id: Ibd1974253e947130fb03a0b86f599e0e5f5eac38