Use implementation instead of api.

This is recommended as a best practice, though AFAIK this only matters
for projects that are going to be imported by other projects.

For instance, you used to be able to include `:deps:extension-api` by
including any extension that depended on it. Now, every project that
wants to use `extension-api` has to include it directly. (Which is fine
because in practice, they all already do so.)

templates/extension/dependencies/android/build.gradle

@@ -17,5 +17,5 @@ android {
 }
 
 dependencies {
-	api project(':deps:extension-api')
+	implementation project(':deps:extension-api')
 }