Remove the tab divider.
Formatting codes.

Replace tab with spaces.
Adding the header bar.
Use Fragment to show each tab content.
Resolve flashing when switch between tabs for first time.

First time switch to a tab, its fragment is created and added, so you
would feel flashing for FragmentTransaction has some latencies. To
resolve flashing, we create and add fragment after setup tabs, so when
switching to a new tab, its fragment is already created, no latency,
thus no flashing.