Replace FragmentTransaction.show/hide with attach/detach - fix lifecycle

[malinajirka]

The recent changes to the BottomNavigation component introduced an anticipated issue with the fragment lifecycle. When show/hide methods are used, even fragments which are not visible on the screen are still in the RESUMED state. This behavior has several downsides - it requires way more memory and also breaks tons of behavior which relies on onPause being invoked when the fragment goes to background. A couple of examples would be tracking and registration to an event bus.

To test:

• Make sure bottom navigation works
• Make sure only a single instance of each fragment lives in the memory
• Make sure onPause is called when you select a different tab

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[planarvoid]

I've checked the behaviour, debugged and profiled the app and it seems to work well 👍 good job