Mattrayner/refactor android execute function

[mattrayner]

Description

• Refactored execute() into multiple helper functions
• Removed some duplication throughout the file
• Tidied up some import statements

  Motivation and Context

The VuforiaPlugin.java file was getting out of control, we'd bolted a lot on to it with little to no consideration of managing the code base. I've split the file into a number of files and removed some duplication and tech debt.

How Has This Been Tested?

On an LG Nexus device running Lollipop

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

  Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I have read the CONTRIBUTING document.

[mattrayner]

@dsgriffin this is mergable

[dsgriffin]

Fixes #61