fix multi gdrive external storage bug

[karakayasemi]

Port of owncloud/core#34987

Description

• Fixes bug on multiple gdrive account addition
• As @Lirein explained in Google Drive external js fix core#31497 (comment). currently oauth2_step1 and oauth_step2 handlers are always registering for the first container in gdrive.js. I moved them inside of whenSelectBackend event and registered each of them separately.

@op6sie, @Lirein please test if you available.

Related Issue

closes #28, closes #24

Motivation and Context

Fixing bugs.

How Has This Been Tested?

The changes tested manually with the following scenarios, all of that worked;

• Add single gdrive account.
• Add multiple gdrive accounts.
• Firstly, add an external owncloud storage and secondly add a gdrive account
• Firstly, add a gdrive account and secondly add an external owncloud storage.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

I used core/master when I test changes. Because of that, I bumped max-version to 11 temporarily in my local, but I did not add this info.xml change to the commit. @PVince81 should I add it?