fix segfault during shell nandroid backup/restore commands

Previously, to achieve a successful "adb shell nandroid backup" and
"adb shell nandroid restore [backup_path]", we have to disable
progress bar by manually removing '/sdcard/clockworkmod/.hidenandroidprogress'

Else, the recovery would crash with segfault

This patch disables progress bar update whenever ui is not initialized

adb shell backup/restore commands now work properly without any background hack
6.44.2