Add special LaunchAppStep handler into executeNestedStep

[zcsteele]

Summary

I have a possible solution to #62 implemented here, which I have verified in the following conditions:

appId: com.google.android.deskclock
onFlowStart:
  - launchApp

onFlowStart:
  - launchApp
      appId: com.google.android.deskclock

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Changes Made

Added a new custom step condition to executeNestedStep for step type *flow.LaunchAppStep. This case block checks to see if AppID is blank, which is true if the launchApp step in the flow does not include an explicit app id.

If the AppID is blank, it is set using the default value from the flow configuration.

Related Issues

Fixes #62

Testing

• Tests pass locally (make test)
• Added tests for new functionality
• Tested manually with sample flows

Checklist

• Code follows project style guidelines
• Self-reviewed the code
• Added/updated documentation as needed
• No breaking changes (or documented if breaking)
• CHANGELOG.md updated (for notable changes)

Additional Notes

I could use some quick pointers on where/how I should add test cases to validate this behavior