test(java): remove duplicate code in BackButtonMultipageTest

[breautek]

This is a recreation of #1038
See #1038 (comment) for why a new PR was created.

This PR contains the original commit authored by @leofernandesmo
I've added a second commit that applies a review suggestion and also mimic the changes in the androidx commit for consistency.

Platforms affected

Android

Motivation and Context

Test code may contain undesirable duplication. Solutions are similar to those for normal code duplication as described by Fowler.
Removing duplication helps maintainability of the test.

Description

I applied extract method and defined constants for duplicate strings.

Testing

Ran npm test

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• 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)
• I've updated the documentation if necessary

[codecov-io]

Codecov Report

Merging #1129 (0ca2fd5) into master (bb7d733) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1129   +/-   ##
=======================================
  Coverage   70.97%   70.97%           
=======================================
  Files          21       21           
  Lines        1747     1747           
=======================================
  Hits         1240     1240           
  Misses        507      507           

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[raphinesse]

LGTM