Use ReadFileToString() in GetHash()

On some devices the previous implementation of GetHash() was failing at
reading files, returning "Bad file descriptor" error.
Using ReadFileToString() that seems to have fixed the issue.

Test: libsnapshot_test
Change-Id: Ie3598d8bad5b3db1cf816c120e6922a066aa2743
Signed-off-by: Alessio Balsini <balsini@google.com>