Specify the algorithm used to sign APK packages (#6404)

Required for compatibility with older versions of Android

packages/flutter_tools/lib/src/commands/build_apk.dart

@@ -129,6 +129,8 @@ class _ApkBuilder {
       '-keystore', keystore.path,
       '-storepass', keystorePassword,
       '-keypass', keyPassword,
+      '-digestalg', 'SHA1',
+      '-sigalg', 'MD5withRSA',
       outputApk.path,
       keyAlias,
     ]);