[FR]: Add android API 21 in CI

[Jaehwa-Noh]

Is there an existing issue for this?

• I have searched the existing issues

Describe the problem

Some code change not cover the lower android APIs and there's no way to check it on CI.

Describe the solution

Add API 21 at CI.

Additional context

In this commit, the android API 23 was removed from CI.
ab78c44
I don't know exactly the history and what happened at that time, but I felt the needs of cover lower API in CI from #1485 (comment) .

If android 21 API was added in CI, we can find crash as early as possible, and can remove the threat.

Code of Conduct

• I agree to follow this project's Code of Conduct