AndroidNotification::VibrateTimingsMillis not operating as documented

### [REQUIRED] Step 2: Describe your environment

* Operating System version: Windows 10 Enterprise
* Firebase SDK version: 2.3.0
* Firebase Product: Firebase Cloud Messaging
* .NET version: 6.0
* OS: Windows 0 Enterprise

### [REQUIRED] Step 3: Describe the problem

The [`AndroidNotification::VibrateTimingMillis`](https://github.com/firebase/firebase-admin-dotnet/blob/2f7c9a90c66dd32737779feb4300020a80638800/FirebaseAdmin/FirebaseAdmin/Messaging/AndroidNotification.cs#L175) property is documented as follows:

> Gets or sets a list of vibration timings in milliseconds in the array to use. The first value in the
array indicates the duration to wait before turning the vibrator on. The next value
indicates the duration to keep the vibrator on. Subsequent values alternate between
duration to turn the vibrator off and to turn the vibrator on.

Today I wanted to send a notification that would vibrate for 500 milliseconds, so I set this property as follows:

```c#
AndroidNotification notification = new
{
DefaultVibrateTimings = false,
VibrateTimingsMillis = new long[] { 0, 500 }
};
```

On the device though, this resulted in a very long vibration, that lasted for several seconds.

After experimenting, sending a value of `{ 0, 1 }` results in a vibration lasting 1 second - I've reviewed the relevant code for this (in [`VibrateTimingsString`](https://github.com/firebase/firebase-admin-dotnet/blob/2f7c9a90c66dd32737779feb4300020a80638800/FirebaseAdmin/FirebaseAdmin/Messaging/AndroidNotification.cs#L175)) and tested it on its own, and the above code results in the value `0s, 0.500000000s` being sent to the API, which appears correct.

I'm not sure if this is a fault on the device I was testing against as it is running a very dated version of Android unfortunately (we're stuck with it due to customer requirements and it's stuck on Android 7.1.1).

PS: I also noticed a few possible micro-optimisations that could be made to some of this new code, such as pre-sizing the list for the `timingsStringList`, I'm not sure if it's worth opening a PR for these changes.