DeskClock: fix setting alarm volume

Minimum volume for a STREAM_ALARM is 1 (or 4, if caller is unprivile-
ged). Substract the minimum volume from Seekbar's value in order to set
proper volume. Also add permission MODIFY_AUDIO_SETTINGS to be able to
set the volume to 1.

Test: Open DeskClock settings, set alarm volume to 1.
      Reopen app, verify volume is still at 1 (minimum).
      Verify alarm volume is at 1 in settings (minimum).
Signed-off-by: Timi Rautamäki <timi.rautamaki@gmail.com>
Change-Id: Icdb5bf7ebc0daefa12edcd28d03c444709cf75c6