Android: make drawer selection work on startup

Now the MainActivity tells the drawer which item is currently
selected.
Android: do not use back stack for horizontal navigation

This follows the Android style guide for how to use Navigation
drawers and will not create such deep back stacks.
Android: alarm summary uses fragment

This avoids duplication lots of code between the alarm
fragment and main activity.
Android: remove unused SystemAlarmActivity

The fragment is now used exclusively.