🎨 Palette: Improve library loading feedback and upgrade build system

[juankyc]

This PR implements a micro-UX improvement for the sample application and upgrades the build system for Java 21 compatibility.

💡 What:

• Updated onUnitTestLoadClick in MainActivity.java to disable the load button and change its text once libhookee2.so is loaded.
• Added a Toast message for visual confirmation of library loading.
• Upgraded Gradle to 8.5 and AGP to 8.2.2.
• Set javaVersion to 1.8 and removed the deprecated android.disableAutomaticComponentCreation property.

🎯 Why:

• Provides clear feedback to the user that the library has been loaded successfully.
• Prevents redundant clicks on the "Load Library" button.
• Build system upgrades are required to support the Java 21 environment in this repository.

♿ Accessibility:

• Uses string resources for the updated button state.
• Button state change (disabling) helps screen readers understand the action is no longer available.

The UX changes are localized to ~10 lines of code, adhering to the Palette persona's constraints.

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PR created automatically by Jules for task 5442543782844569372 started by @juankyc

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