[firebase_storage] Fix Content-Type auto-detection for Android

[long1eu]

Description

Fix auto-detection of Content-Type for Android.

Related Issues

flutter/flutter#32181

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Breaking Change

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[long1eu]

A new issue that I found was related to the leading / on Android.

I/flutter ( 6858): 00:00 +0: FirebaseStorage putFile, getDownloadURL, writeToFile [E]
I/flutter ( 6858):   Expected: 'text/foo44c6da40-8080-11e9-eab5-7d515fb499d2.txt'
I/flutter ( 6858):     Actual: '/text/foo44c6da40-8080-11e9-eab5-7d515fb499d2.txt'
I/flutter ( 6858):      Which: is different.
I/flutter ( 6858):             Expected: text/foo44 ...
I/flutter ( 6858):               Actual: /text/foo4 ...
I/flutter ( 6858):                       ^
I/flutter ( 6858):              Differ at offset 0
I/flutter ( 6858):   
I/flutter ( 6858):   package:test_api/src/frontend/expect.dart 152:30                                                                      fail
I/flutter ( 6858):   package:test_api/src/frontend/expect.dart 146:3                                                                       _expect
I/flutter ( 6858):   package:test_api/src/frontend/expect.dart 59:3                                                                        expect
I/flutter ( 6858):   package:flutter_test/src/widget_tester.dart 212:3                                                                     expect
I/flutter ( 6858):   Users/long1eu/pull_requests/plugins/plugins/packages/firebase_storage/example/test_driver/firebase_storage.dart 31:7  main.<fn>.<fn>
I/flutter ( 6858):   ===== asynchronous gap ===========================
I/flutter ( 6858):   dart:async/future_impl.dart 22:43                                                                                     _Completer.completeError
I/flutter ( 6858):   dart:async-patch/async_patch.dart 40:18                                                                               _AsyncAwaitCompleter.completeError
I/flutter ( 6858):   Users/long1eu/pull_request
I/flutter ( 6858): 00:00 +0 -1: (tearDownAll)
I/flutter ( 6858): 00:00 +1 -1: Some tests failed.

Since documentation for both Android and iOS show images/space.jpg as a result from getPath() I suggest we drop the leading /.

https://firebase.google.com/docs/storage/android/create-reference#reference_properties
https://firebase.google.com/docs/storage/ios/create-reference#reference_properties

[long1eu]

The Android Firebase SDK uses the ContentResolver#getType method to get the MIME type as you can see here.

The issue is that ContentResolver#getType will return null if the scheme is not content://. We could use FileProvider to convert file:/// to content://.

[long1eu]

Another approach, would be to get the Content-Type ourselves and set it on metadata if the user didn't set it already. This approach is also used by the Firebase Storage iOS SDK.

[collinjackson]

Thanks! You've fixed two inconsistencies between iOS and Android with this PR, and the Android integration test now passes.