Fix some cases when refresh rate switching didn't work/timed out (#901)

## Description
An attempt to fix #769

1. Always fetch the `DisplayManager` when switching refresh rates
2. Use the Activity context instead of Application context

### Related issues
Fixes #769

### Testing
I still haven't reproduced #769 myself, but this change works fine on my
shield + LG C2

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/MainActivity.kt

@@ -409,7 +409,7 @@ class MainActivity : AppCompatActivity() {
         }
 
     fun changeDisplayMode(modeId: Int) {
-        lifecycleScope.launch(Dispatchers.Main + ExceptionHandler()) {
+        lifecycleScope.launch(Dispatchers.Main + ExceptionHandler(autoToast = true)) {
             val attrs = window.attributes
             if (attrs.preferredDisplayModeId != modeId) {
                 Timber.d("Switch preferredDisplayModeId to %s", modeId)