Mitigate device info main queue modules crash (#51662)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51662

## Changes
DeviceInfo: Deafult fontScale to 1 when accessibility manager isn't available

# Analysis
**The problem:** For reasons that we don't understand, accessibility manager is sometimes nil, when deviceinfo needs it. This is a long-standing issue in react native.

```
  _constants = @{
    @"Dimensions" : [self _exportedDimensions],
    // Note:
    // This prop is deprecated and will be removed in a future release.
    // Please use this only for a quick and temporary solution.
    // Use <SafeAreaView> instead.
    @"isIPhoneX_deprecated" : @(RCTIsIPhoneNotched()),
  };
```

```
- (NSDictionary *)_exportedDimensions
{
  RCTAssert(!_invalidated, @"Failed to get exported dimensions: RCTDeviceInfo has been invalidated");
  RCTAssert(_moduleRegistry, @"Failed to get exported dimensions: RCTModuleRegistry is nil");
  RCTAccessibilityManager *accessibilityManager =
      (RCTAccessibilityManager *)[_moduleRegistry moduleForName:"AccessibilityManager"];
  if (!accessibilityManager) {
    return nil;
  }
  CGFloat fontScale = accessibilityManager ? accessibilityManager.multiplier : 1.0;
  return RCTExportedDimensions(fontScale);
}

```

**The crash:** When accessibility manager is nil, device info tries to insert nil into an NSDictionary, and crashes.

We found a possible repro of this issue: [launch facebook, log out, and log back in](https://www.internalfb.com/diff/D72020801?transaction_fbid=982516293994114).

**Why this surged now:** Recently, we started initializing device info during react native init. That means this code-path just runs much more often.

# Mitigation
Remove the return nil in `[self _exportedDimensions]`. Instead, just default the fontScale to 1 when the accessibility manager isn't available.

**This should be safe:** If we assume that accessibility manager eventually becomes available, the fontScale will eventually become correct. Device info will send the updated fontScale to js when it becomes available: [native](https://www.internalfb.com/code/fbsource/[ec6fd664a9cd]/xplat/js/react-native-github/packages/react-native/React/CoreModules/RCTDeviceInfo.mm?lines=231-232), [js](https://www.internalfb.com/code/fbsource/[ec6fd664a9cd]/xplat/js/react-native-github/packages/react-native/Libraries/Utilities/Dimensions.js?lines=120-125).

# Long-term fix
Understand why accessibility manager is nil.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D75537894

fbshipit-source-id: 921cc573fdfd7e5c340ac3a4ada268caadb9e382