Bug 1277407 - Add Adjust sandbox keyfile to tree. r=sebastian Additionally, we add this file to the tree so it can be used by bug 1277553. The following commit will add the docs explaining how to use the file I added. The added file contents are slurped directly into AdjustConstants.MOZ_INSTALL_TRACKING_ADJUST_SDK_APP_TOKEN. We get the sandbox token from the previously working value in [1]. An alternative solution would be to remove the assertion that `--with-adjust-sdk-keyfile` is specified and provide a default value if the key is not specified. However, after investigating bug 1277553, I dislike this approach because we lose the compile-time checks that a key file is specified, which is dangerous for release/beta builds where the key file may not be valid, we push a release, and we don't get the data we expect. Ideally, we specify `--with-adjust-sdk-keyfile` for non-MOZILLA_OFFICIAL builds, but I don't know how to do that. [1]: https://hg.mozilla.org/mozilla-central/rev/ac75f8f4c01d#l1.26 MozReview-Commit-ID: HPNgiwfzU5p

new file mode 100644
--- /dev/null
+++ b/mobile/android/base/adjust-sdk-sandbox.token
@@ -0,0 +1,1 @@
+ABCDEFGHIJKL