Apply flowOn(defaultDispatchers) for main-safe.

[Jaehwa-Noh]

What I have done and why
I've added withContext(defaultDispatcher) for main-safe at combine.

Fix #1420

DefaultSearchContentsRepository.searchContents

Before	After

DefaultSearchContentsRepository.getSearchContentsCount

Before	After

GetFollowableTopicsUseCase.invoke

Before	After

Flow.mapToUserSearchResult

Before	After

[hoc081098]

How about

combine(a, b) { ... }
    .flowOn(defaultDispatcher)

I think the above code has the same meaning.

[Jaehwa-Noh]

combine(a, b) { ... }
    .flowOn(defaultDispatcher)

Good suggestion. I'll accept your suggestion.
Thank you.

[dturner]

Getting a build error:

e: file:///home/runner/work/nowinandroid/nowinandroid/core/data/src/test/kotlin/com/google/samples/apps/nowinandroid/core/data/CompositeUserNewsResourceRepositoryTest.kt:40:9 No value passed for parameter 'defaultDispatcher'

[Jaehwa-Noh]

#1250 This issue had given the clue that how to solve the test fail problems when I was struggling with some tests fail.
The key was single scheduler!

Thanks.