AWS calls should specify json response

[mmcc007]

Modify aws calls to use json in response

This command doesn't work for me as well : sylph -d android | grep Pixel

sylph -d android | grep Pixel
Unhandled exception:
FormatException: Missing expected digit (at character 2)
---------------------------------------------------------------------------...
 ^

#0      _ChunkedJsonParser.fail (dart:convert-patch/convert_patch.dart:1394:5)
#1      _ChunkedJsonParser.parseNumber (dart:convert-patch/convert_patch.dart:1258:9)
#2      _ChunkedJsonParser.parse (dart:convert-patch/convert_patch.dart:926:22)
#3      _parseJson (dart:convert-patch/convert_patch.dart:31:10)
#4      JsonDecoder.convert (dart:convert/json.dart:495:36)
#5      JsonCodec.decode (dart:convert/json.dart:153:41)
#6      jsonDecode (dart:convert/json.dart:96:10)
#7      deviceFarmCmd (package:sylph/src/base/utils.dart:150:10)
#8      getDeviceFarmDevices (package:sylph/src/device_farm.dart:290:30)
#9      getDeviceFarmDevicesByType (package:sylph/src/device_farm.dart:283:10)
#10     run (file:///Users/angela/flutter/.pub-cache/hosted/pub.dartlang.org/sylph-0.7.2/bin/main.dart:76:32)
#11     main.<anonymous closure> (file:///Users/angela/flutter/.pub-cache/hosted/pub.dartlang.org/sylph-0.7.2/bin/main.dart:62:13)
#12     AppContext.run.<anonymous closure> (package:tool_base/src/base/context.dart:154:29)
#13     _rootRun (dart:async/zone.dart:1126:13)
#14     _CustomZone.run (dart:async/zone.dart:1023:19)
#15     _runZoned (dart:async/zone.dart:1518:10)
#16     runZoned (dart:async/zone.dart:1465:12)
#17     AppContext.run (package:tool_base/src/base/context.dart:153:18)
#18     runInContext (package:sylph/src/context_runner.dart:11:24)
#19     main (file:///Users/angela/flutter/.pub-cache/hosted/pub.dartlang.org/sylph-0.7.2/bin/main.dart:61:11)
#20     _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:299:32)
#21     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

Originally posted by @AnzhelaAninian in #95 (comment)