Allow disabling GL preload even if gfxdriver isn't updatable

Bug: 34611670
Test: set ro.zygote.disable_gl_preload=1, shell start,
      confirm GL driver isn't loaded in zygote process
      by checking /proc/<pid>/maps

Change-Id: Ieb739bd7eba365917684cf830d78c15fea06f8cf
(cherry picked from commit 5d911fe6)