Use declarative plugin block for flutter's gradle plugin

[vyPal]

Description

Makes the app use the new declarative plugin block for adding flutter's gradle plugin

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Test coverage

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules