fix: prevent reaching the congratulations screen during installation

[imobachgs]

Problem

Agama determines in which point of the installation it is by the combination of two values: the
installation phase (startup, config and install) and the isBusy flag. This method has proven to be
rather fragile.

One of the typical issues is jumping into the "congratulations" screen
when the installation is not even finished. See #1616.

Although we mitigated this problem, it recently appear in our QA testing.

Solution

Let's introduce a new phase, "finish", which indicates that the installation is finished. As simple
as that. So we do not need a combination of two values anymore at that point. We plan to extend this
approach to other parts.

Testing

• Added a new unit test
• Tested manually

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Related to https://trello.com/c/UmVLEYxE (internal link)

[coveralls]

Pull Request Test Coverage Report for Build 12947364045

Details

• 8 of 9 (88.89%) changed or added relevant lines in 3 files are covered.
• 20 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 70.911%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rust/agama-lib/src/manager.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
rust/agama-lib/src/manager.rs	1	0.0%
service/lib/agama/dbus/manager.rb	6	92.31%
service/lib/agama/manager.rb	13	80.99%

Totals
Change from base Build 12946688667:	0.02%
Covered Lines:	17264
Relevant Lines:	24346

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💛 - Coveralls

[dgdavid]

For the record, by adding the FINISHED installation phase, this finally solves (in a more robust way) the problem previously reported at #1616 and mitigated at #1690.