TokencodeBackend: Don't guess at initial screen-on state

Set mScreenOn from PowerManager instead of assuming "true" until we get
our first ACTION_SCREEN_OFF/ACTION_SCREEN_ON broadcast.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
MainActivity: Unbreak DevidFragment for the "token not present" case

Commit cf2c1c3396 (Don't draw divider if only frag_0 is present)
introduced a regression that caused mTokenPresent to be set to true
for both present and not-present cases.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>