Update sample to call preload at a more appropriate time

[kiftio]

What changes are you making?

The README states that preload() should be used responsibly as it results in background network requests and additional CPU/memory utilization for the client. And that it should only be used when there is a high likelihood that the buyer will soon request to checkout.

But the sample apps call it on each cart mutation / add to cart, which is more frequently than it needs to be called.

This updates the sample to follow the README advice, and only call preload() when a customer lands on the cart page.

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It also updates an incorrect line r.e. min JDK version in the README

How to test

Click around in the sample

Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines.
• I have read and agree with the Code of Conduct.
• I've updated the README.

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Checklist for releasing a new version

• I have bumped the version number in the build.gradle file
• I have added a Changelog entry.
• I have updated the versions in the README.md for both Gradle and Maven.

Tip

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