Use Option::insert() (#729)

## Type of change

<!-- (mark with an `X`) -->

```
- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other
```

## Objective
Just discovered that `Option::insert` is a thing, which simplifies some
code that first sets the value and then immediately returns a reference
to it.

crates/bitwarden/src/client/client.rs

@@ -253,12 +253,11 @@ impl Client {
         user_key: EncString,
         private_key: EncString,
     ) -> Result<&EncryptionSettings> {
-        self.encryption_settings =
-            Some(EncryptionSettings::new(master_key, user_key, private_key)?);
-        Ok(self
-            .encryption_settings
-            .as_ref()
-            .expect("Value is initialized previously"))
+        Ok(self.encryption_settings.insert(EncryptionSettings::new(
+            master_key,
+            user_key,
+            private_key,
+        )?))
     }
 
     #[cfg(feature = "internal")]
@@ -267,14 +266,12 @@ impl Client {
         user_key: SymmetricCryptoKey,
         private_key: EncString,
     ) -> Result<&EncryptionSettings> {
-        self.encryption_settings = Some(EncryptionSettings::new_decrypted_key(
-            user_key,
-            private_key,
-        )?);
         Ok(self
             .encryption_settings
-            .as_ref()
-            .expect("Value is initialized previously"))
+            .insert(EncryptionSettings::new_decrypted_key(
+                user_key,
+                private_key,
+            )?))
     }
 
     #[cfg(feature = "mobile")]
@@ -292,10 +289,8 @@ impl Client {
         &mut self,
         key: SymmetricCryptoKey,
     ) -> &EncryptionSettings {
-        self.encryption_settings = Some(EncryptionSettings::new_single_key(key));
         self.encryption_settings
-            .as_ref()
-            .expect("Value is initialized previously")
+            .insert(EncryptionSettings::new_single_key(key))
     }
 
     #[cfg(feature = "internal")]