Fix un-selection background color on Android DetailedList and Table

[proneon267]

Originally identified in #3118 and as suggested by @mhsmith, this PR(independent of #3118) fixes the following bug which is present in DetailedList and Table on Android:

        toga.Box(
            style=Pack(flex=1, background_color="#87CEFA"),
            children=[
                toga.DetailedList(...)
            ],
        )

Reason for not enabling background color tests in this PR: #3015

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct