[dart:io] Use /system/bin/sh on Android.

TEST=standalone/io/regress_flutter_57125_test
Change-Id: Ief5619ffe26f85bab28ad7ab16db835d11814512
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438263
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

sdk/lib/_internal/vm/bin/process_patch.dart

@@ -327,6 +327,9 @@ base class _ProcessImpl extends _ProcessImplNativeWrapper implements _Process {
     if (Platform.isWindows) {
       return 'cmd.exe';
     }
+    if (Platform.isAndroid) {
+      return '/system/bin/sh';
+    }
     return '/bin/sh';
   }