GH-122 android: Cleanup Dialog plugin code

[hazems]

Platforms affected

Android

Description

1. Organizing Notification imports.
2. Externalizing Action strings in private static finals.
3. Externalizing Beep timeout and wait time in private static finals.

Testing

Test plugin and everything works fine.

• I've run the tests to see all new and existing tests pass
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)