android: Lower minSdkVersion to 26, aka Android 8 Oreo

Fixes zulip#1898.

Author: gnprice

android/app/build.gradle

@@ -35,7 +35,7 @@ android {
 
     defaultConfig {
         applicationId "com.zulipmobile"
-        minSdkVersion 28
+        minSdkVersion 26
         targetSdkVersion flutter.targetSdkVersion
         // These are synced to local.properties from pubspec.yaml by the flutter tool.
         versionCode localProperties.getProperty('flutter.versionCode').toInteger()