Move SingletonTest back to libutils_test

Use the new "data_libs:" to specify test library dependency. This is
like "data:" and it treats compiled libraries as test data files.
The result is libutils_test_singleton{1,2} is picked up by libutils_test
and installed next to the test binary.

Mark the test libs libutils_test_singleton{1,2} as uninstallable. This
prevents installing libutils_test_singleton{1,2} directly and only
permits installing via "data_libs:".

```
$ m libutils_test
testcases
└── libutils_test
    ├── arm
    │   ├── libutils_test
    │   ├── libutils_test_singleton1.so
    │   └── libutils_test_singleton2.so
    ├── arm64
    │   ├── libutils_test
    │   ├── libutils_test_singleton1.so
    │   └── libutils_test_singleton2.so
    └── libutils_test.config
```

Bug: 124838889
Test: atest libutils_test
Change-Id: I432135e128fc9eedb1b8c18a331957e271d8b0f0