Avoid a race condition on reading qemu.gles property

The qemu.gles property will be ready after the SurfaceFlinger service is
initializing. Once the all required variable is set, it caches the
result for the future calls.

Bug: 37759781
Test: Test with imx6ul and imx7d pico boards.
Change-Id: Ib9f11a51ec8a0707be854f67fc8591149ab7fd49
2 files changed