Changed robot frame base_link to base_footprint

[FlorGrosso]

Changed robot frame from base_link to base_footprint, in file strings.xml.

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[FlorGrosso]

I signed it!

[googlebot]

CLAs look good, thanks!

[adamantivm]

@FlorGrosso thank you! But could you please remove the gradle wrapper files, and only submit the change to the resources file? Thanks!

[FlorGrosso]

@adamantivm comment addressed, ready for another pass.

[adamantivm]

Thanks @FlorGrosso !