Fix missing experimental annotations

[chris-mitchell]

The PR #289 didn't seem to run on CI and missed some checks.

Two main issues were introduced:

• It removed @ExperimentalCoroutinesApi annotations in tests which are needed due to https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/run-blocking-test.html still having the annotation.
• It removed some @ExperimentalTime annotations which are still experimental in Kotlin 1.5 - https://kotlinlang.org/docs/whatsnew15.html#new-api-for-getting-a-char-category-now-available-in-multiplatform-code

This reverts only the appropriate pieces and leaves most of the changes in place.

[codecov]

Codecov Report

Merging #305 (0502d03) into main (555fe5d) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 0502d03 differs from pull request most recent head 3cf66e2. Consider uploading reports for the commit 3cf66e2 to get more accurate results

@@             Coverage Diff              @@
##               main     #305      +/-   ##
============================================
+ Coverage     85.33%   85.36%   +0.02%     
+ Complexity      180      178       -2     
============================================
  Files            49       49              
  Lines           914      909       -5     
  Branches        126      126              
============================================
- Hits            780      776       -4     
+ Misses           77       76       -1     
  Partials         57       57              

Impacted Files	Coverage Δ
...va/com/dropbox/android/external/fs3/FSAllReader.kt	100.00% <ø> (ø)
...dropbox/android/external/fs3/SourceAllPersister.kt	0.00% <ø> (ø)
...box/android/external/fs3/SourcePersisterFactory.kt	0.00% <ø> (ø)
...otlin/com/dropbox/flow/multicast/ChannelManager.kt	89.06% <ø> (ø)
...n/kotlin/com/dropbox/flow/multicast/Multicaster.kt	96.96% <ø> (-0.09%)	⬇️
...2/src/main/kotlin/com/dropbox/store/rx2/RxStore.kt	100.00% <ø> (ø)
...ain/kotlin/com/dropbox/store/rx2/RxStoreBuilder.kt	100.00% <ø> (ø)
...3/src/main/kotlin/com/dropbox/store/rx3/RxStore.kt	100.00% <ø> (ø)
...ain/kotlin/com/dropbox/store/rx3/RxStoreBuilder.kt	100.00% <ø> (ø)
...om/dropbox/android/external/store4/StoreBuilder.kt	100.00% <ø> (ø)
... and 4 more

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