1. Use vkMergePipelineCaches to recreate pipeline cache after
   blob cache callbacks are set after eglInititalize.
2. Use a more proper way to save the cache data to disk.

Bug: angleproject:3318
Change-Id: Ied1fb572813198b51e02ed9629cbf34e2d9159b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1683807
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>