fix(android): get external files directory for Android 10+

[erisu]

Platforms affected

Android

Motivation and Context

To resolve mounted storage for Android 11.
Android 11 (API 30) has deprecated the method Environment.getExternalStorageDirectory().

Description

This PR has replaced the Environment.getExternalStorageDirectory() usage with context.getExternalFilesDir(null).

Per Android documentation, getExternalFilesDir is one of the suggested alternative solutions.

PR also includes a little bit of refactoring to remove code duplication that related to the calling of getExternalStorageDirectory, now getExternalFilesDir.

Testing

• In-Progress...

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[breautek]

lgtm -- later tonight I'll try to give it an actual test

[retoko]

From google docs:

Starting in November 2021, app updates will be required to target API level 30 or above

What needs to release this fix? any help?

[erisu]

Last call for reviews. This PR will be merged in tomorrow (JST).