Fix #6228: Crash in Android dialog module.

[dryganets]

Motivation

Android dialog module has a race condition as a result of which it crashes.
See this issue:
#6228.

The mIsInForeground flag is set on UI thread but was used from the ReactMethod thread.
Now all public methods of FragmentManagerHelper called from UI thread only.
Asserts are added in appropriate places to prevent future regressions.

Test Plan

Make sure that dialogs work after this change.
It will be nearly impossible to reproduce the issue manually but automatic regression tests should be able to catch this. At least our tests were crashing on some dialog scenarios from time to time.

Release Notes

[ANDROID] [MINOR] [BUGFIX] - Race condition fix in Android Dialogs module.

[dryganets]

The failing test is unrelated to the change:

#!/bin/bash -eo pipefail
buck fetch ReactAndroid/src/test/java/com/facebook/react/modules
Not using buckd because watchman isn't installed.
Picked up _JAVA_OPTIONS: -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap
PARSING BUCK FILES... 1.3s (100%)
BUILD FAILED: Buck wasn't able to parse /home/circleci/react-native/ReactCommon/yoga/BUCK:
NameError: name 'fb_xplat_cxx_library' is not defined
Call stack:
  File "/home/circleci/buck/python-dsl/buck_parser/buck.py", line 1210, in process_with_diagnostics
    diagnostics=diagnostics)
  File "/home/circleci/buck/python-dsl/buck_parser/buck.py", line 1101, in process
    os.path.join(self._project_root, path))
  File "/home/circleci/buck/python-dsl/buck_parser/buck.py", line 1095, in _process_build_file
    return self._process(build_env, path, is_implicit_include=False)
  File "/home/circleci/buck/python-dsl/buck_parser/buck.py", line 1047, in _process
    exec(code, module.__dict__)
  File "/home/circleci/react-native/ReactCommon/yoga/BUCK", line 1
    fb_xplat_cxx_library(


This error happened while trying to get dependency '//ReactCommon/yoga:yoga' of target '//ReactAndroid/src/test/java/com/facebook/react/modules:modules'
Exited with code 1

[facebook-github-bot]

@mdvacca has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.