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We now detect at runtime which sync features to use, which
allows us to remove a lot of the compile-time configuration
options. There  is still one option though, to disable
KHR_fence_sync on some devices (which are more efficient
without it).

- added a backdoor to get the vendor's EGL strings

the new logic is:
- use always ANDROID_native_fence_sync if available
- fallback to KHR_fence_sync if available and not disabled
  by the compile-time option
- use KHR_wait_sync if available and either of the above is
  enabled

Change-Id: I9c4b49d9ff1151faf902cc93bd53ea5f205aaabf