set preloading tag on instrumentation event

What changes are you making?

Update instrumentation to pass through a preloading tag. Basing the value on whether the last loadCheckout has isPreload = true

How to test

Open checkout, preload, check instrumentation payload. Do the same without preloading

Before you merge

[!IMPORTANT]

[x] I've added tests to support my implementation

[x] I have read and agree with the Contribution Guidelines.

[x] I have read and agree with the Code of Conduct.

[ ] I've updated the README.

Checklist for releasing a new version

- [ ] I have bumped the version number in the [`build.gradle` file](https://github.com/Shopify/checkout-kit-android/blob/main/lib/build.gradle#L17) - [ ] I have added a [Changelog](https://github.com/shopify/checkout-sheet-kit-android/blob/main/CHANGELOG.md) entry.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.

Shopify / checkout-sheet-kit-android