Add API 21 to CI.

Change-Id: I7e1406da9bf7d8aca44f48d9d7dfeb123d67a562

Author: Jaehwa-Noh

.github/workflows/Build.yaml

@@ -153,7 +153,7 @@ jobs:
     timeout-minutes: 55
     strategy:
       matrix:
-        api-level: [26, 30]
+        api-level: [21, 26, 30]
 
     steps:
       - name: Delete unnecessary tools 🔧