Revert "Use snackbar lib from maven repo"

This reverts commit bfc57aefa5.

Conflicts:
	OpenKeychain/build.gradle
	settings.gradle

.gitmodules

@@ -22,3 +22,6 @@
 	path = extern/safeslinger-exchange
 	url = https://github.com/open-keychain/exchange-android
 	ignore = dirty
+[submodule "extern/snackbar"]
+	path = extern/snackbar
+	url = https://github.com/open-keychain/snackbar