1. Use vkMergePipelineCaches in eglSetBlobCacheFuncsANDROID as
   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: Ieb5d10ab93e7afb2aab4446b387d7f36c878a686
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1559671Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>