RN: iOS - Channel spinner is black on dark theme

Description

On iOS 13 with light mode, the channel spinner is black when using a dark theme.

Steps to repro:
1. Ensure light mode is set on your iOS device
2. Log in to the iOS app
3. Set the theme to either Mattermost Dark or Windows Dark
4. Return to the Channel screen
5. Swipe up to trigger loading and notice spinner is black.

QA Test Steps

None

Status

Mana

2

Assignee

Unassigned

QA Assignee

None

Reporter

Miguel Alatzar

Epic Link

None

Fix versions

Mattermost Team

Mobile

Sprint

None

Labels

None

QA Testing Areas

Mobile - RN

GitHub Issue

None

Components

None
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