[TIMOB-26488] Android: Fix Ti.Filesystem.requestStoragePermissions for Android 8 and above

[ypbnv]

JIRA: https://jira.appcelerator.org/browse/TIMOB-26488

Description:
In Android 8 Google fixed a problem where granting a READ_EXTERNAL_STORAGE permissions automatically granted all the permissions in the group. This PR adds the WRITE_EXTERNAL_STORAGE permissions in the requestStoragePermissions() method in Filesystem module in order to not break existing apps relying on it providing write permission.

No unit test for this PR.

Test case:

This fix is only required for Android 8 and above.
Make sure you have both READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE declared in tiapp.xml
You can use any image placed in the Resources folder of the application.
After receiving a success callback you can check the devices Gallery\Photos application for the image. It may take some time (like 10 seconds) to appear there.
Before this PR you will always get the error callback.

app.js

let win = Ti.UI.createWindow({
        layout: 'vertical'
    }),
    permissionsButton = Ti.UI.createButton({
        title: 'Request permissions'
    }),
    addToGalleryReel = Ti.UI.createButton({
        title: 'Add to gallery reel',
        enabled: false,
        touchEnabled: false
    });

win.addEventListener('open', function () {
    addToGalleryReel.enabled = Ti.Filesystem.hasStoragePermissions();
    addToGalleryReel.touchEnabled = Ti.Filesystem.hasStoragePermissions();
});

permissionsButton.addEventListener('click', function () {
    if (!Ti.Filesystem.isExternalStoragePresent()) {
        alert('No external storage available.');
        return;
    }
    if (Ti.Filesystem.hasStoragePermissions()) {
        alert('Permissions already granted.')
    } else {
        Ti.Filesystem.requestStoragePermissions( function (requestCallback) {
            addToGalleryReel.enabled = requestCallback.success;
            addToGalleryReel.touchEnabled = requestCallback.success;
        });
    }
});

addToGalleryReel.addEventListener('click', function () {
    let file = Ti.Filesystem.getFile(Ti.Filesystem.resourcesDirectory, <your-image-here>);
    Ti.Media.saveToPhotoGallery(file, {
        success: function() { alert('Success.') },
        error: function() { alert('Error.') }
    });
});

win.add([permissionsButton, addToGalleryReel]);
win.open();

[garymathews]

CR: PASS

[drauggres]

https://developer.android.com/reference/android/Manifest.permission#READ_EXTERNAL_STORAGE

Any app that declares the WRITE_EXTERNAL_STORAGE permission is implicitly granted this permission.

So requesting only WRITE_EXTERNAL_STORAGE should be enough

[longton95]

FR Passed

Tested using the tescase above, the permissions are requested succcessfully and the image is saved.

ENV

Nexus 6P (8.1.0)
Android Emulator (8.0, 8.1.0, 9.0)
macOS: 10.14.1
Windows 10 Pro: 1703
Appc NPM: 4.2.13
Appc CLI: 7.0.7
Ti CLI: 5.1.1
Node: 8.12.0
NPM: 6.4.1

Awaiting Jenkins

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