Avoid to call UI functions in child process

ui_print uses a mutex to protect the critical section. And a forked
child process may intialize the mutex inappropriatly, which leads to a
hanging child process. So we shall avoid to call the UI functions in
child process.

Bug: 34769056
Test: fake ota on fugu failed as expected due to updater format mismatch.
Change-Id: I81bc942a796878fac19cf712512092c2b0d807c9