Update examples to explicitly list gradle dependencies.

[freakboy3742]

With the introduction of beeware/briefcase#1610, the Toga examples need to explicitly list their gradle dependencies to avoid a warning at time of build.

The testbed and hardware app have been updated as part of #2350.

Testing the hardware app revealed that there is a discrepancy between the explicitly declared App ID and the app ID constructed from the bundle and app name; I've used this opportunity to normalise all the examples into an org.beeware.toga.examples namespace.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct