Just tested on emulator version 21 and seems to work, so loosen restrictions

app/build.gradle

@@ -15,7 +15,7 @@ android {
     buildToolsVersion "29.0.3"
     defaultConfig {
         applicationId "com.geeksville.mesh"
-        minSdkVersion 22 // The oldest emulator image I have tried is 22 (though 21 probably works)
+        minSdkVersion 21 // The oldest emulator image I have tried is 22 (though 21 probably works)
         targetSdkVersion 29
         versionCode 175
         versionName "0.7.5"