Make app_import_test.go compatible with future golang version

https://go-review.git.corp.google.com/c/tools/+/585795 makes it an error
to have a non constant string as the sole argument to `Errorf`.

Test: go version # 1.24
Test: go test ./java
Change-Id: Ic060535cb3ee0fe040efc82421911d290eecb9a2