Fix crash in onSaveInstanceState in Reader

[malinajirka]

Fixes #11992

As explained here this issue was introduced with the changes to the BottomNavigation.

However, the root cause of this issue was introduced by Google in AppCompat 1.1.0. onSaveInstanceState is invoked even for fragments on back stack which views were never displayed/created. More info here.

Downside is that we'll lose scroll position on double rotation, but I don't think it's a big deal. Especially since this will happen only when the user isn't on the Reader tab.

To test:

1. Open the App
2. Click on Reader
3. Click on "My Site"
4. Rotate your device twice -> make sure the app doesn't crash

---

1. Enable "Don't keep activities" under Device Settings -> Systems -> Advanced -> Developer options -> Apps section
2. Open WordPress Android app and toggle from Reader tab to My site tab
3. Click Blog Posts on My site tab and return from it
4. Click Blog Posts again or Site Pages on My site tab -> make sure the app doesn't crash

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.

This crash was introduced in 14.9-rc-1, so this PR is targeting a frozen branch (cc @oguzkocer).

[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.

[ashiagr]

LGTM, 👍.