[Filesystem] Fixed implementation of readAsText().

[Filesystem] Fixed implementation of readAsText().

Read operation is now performed asynchronously on main thread.
Recursive call removed.

[Verification] Pass rate: 289/289.
Warning: Due to nature of implementation and current architecture, the test
File_readAsText_with_encoding may block. If there are large files present in
/opt/usr/media/Documents/ folder (i.e. created by Download test suite) the test
will not be able to finish in time.

Change-Id: I031ac1d7c6d3ab223a0f93a0788938d5067b8a8c
Signed-off-by: Pawel Andruszkiewicz <p.andruszkie@samsung.com>