Zygote: Fix GL preload property evaluation

Commit b3a14176 ("Zygote: Restore GL preload")
should allow to disable GL preloading on devices with no updatable
drivers support (i.e. empty ro.gfx.driver.0). However, the property
ro.zygote.disable_gl_preload has no effect on a device with no
updatable driver support and it does on a device with updatable
driver support, which is the opposite of what we want.

Change-Id: I3df6c929a465441632ca1540dbb575daa174b0fb