Moved `isFileExist` method to IO thread. #2644

Job	Run time
Automated tests for Custom app (25)	11m 10s
Automated tests (25)	1h 22m 11s
Automated tests for Custom app (30)	9m 16s
Automated tests for PlayStore variant (25)	13m 45s
Automated tests on Tablet (30)	7m 36s
Automated tests (30)	1h 22m 12s
Automated tests for Custom app (33)	9m 19s
Automated tests for PlayStore variant (30)	9m 27s
Automated tests on Tablet (33)	8m 2s
Automated tests for Custom app (34)	10m 1s
Automated tests (33)	1h 4m 51s
Automated tests for PlayStore variant (33)	11m 6s
Automated tests on Tablet (34)	7m 41s
Automated tests (34)	1h 22m 8s
Automated tests for PlayStore variant (34)	9m 43s
	6h 58m 28s

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