SurfaceFlinger: add a sysprop for GL backpressure

In some devices where SF misses a frame in GL comp, HWC queue may
get stuffed. This causes application to block longer on dequeueBuffer
which leads to jank.

Test: 1. Open Camera
      2. Swipe horizontally to go back to previous app.

Bug: 138083790
Change-Id: I4c5963cb7ba7f9b10254c77dc7117ca3acac81cf