Added missing meta files.

[neogeek]

Two new files were recently added to this repo without their .meta counterparts. This PR adds those missing files.

• Runtime/Android/THIRD_PARTY_NOTICES.txt
• Runtime/iOS/THIRD_PARTY_NOTICES.txt

Without the .meta files, after installing the latest package, you will get the following errors:

Asset Packages/com.google.xr.cardboard/Runtime/Android/THIRD_PARTY_NOTICES.txt has no meta file, but it's in an immutable folder. The asset will be ignored.
Asset Packages/com.google.xr.cardboard/Runtime/iOS/THIRD_PARTY_NOTICES.txt has no meta file, but it's in an immutable folder. The asset will be ignored.

I tried submitting a Contributor License Agreement, but submitting the form resulted in a 400 error.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[rjcausarano]

Thanks for your pull request. We have started internal review. Please don’t push any more changes into this PR as they might be overwritten.