android: Lower minSdkVersion to 26, aka Android 8 Oreo

Fixes zulip#1898.

In general, our policy for supporting old OS versions is
described here (in the legacy app's repo, but it applies equally
to this app):
  https://github.com/zulip/zulip-mobile/blob/eb8505c4a/docs/architecture/platform-versions.md

So for versions old enough that they represent a very small fraction
of our overall user base, we don't have the resources to actively
maintain support; but if the app works on those versions, we leave
them supported in the app's metadata.

When we increased the minimum to Android 9, the reason was that we
weren't sure whether things would break on older Android versions,
and just didn't have a device older than Android 9 to test on:
  https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/flutter.3A.20Java.20versions.3F/near/1775991

But in zulip#1898 we've learned that the app does in fact work on
Android 8.  So restore that support in the app's manifest.

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()