Fix race condition in extension string creation

Change a17429660467548a1bc5f8d435a327500fbe972b introduced a race condition where the notfiy_all is called before the mExtensionString is set.
Set mExtensionString before refCond.notify_all(); is called

Bug: 345419965
Bug: 346881664
Test: Ran EGL unit tests
(cherry picked from https://android-review.googlesource.com/q/commit:5e94634dc57024dcae2d1484758c2a9a7a55d55e)
Merged-In: Ifcb691f9b9769be494e204c97512c3f7fffec60a
Change-Id: Ifcb691f9b9769be494e204c97512c3f7fffec60a
1 file changed