Back button from settings screen should return to HomeTab

[agzuniverse]

Back button currently closes the app if it's used from the settings screen as it is part of the tab navigator. This is however a rather unexpected behaviour.

Tried fixing it by #2710 which set the backBehavior to initalRoute (which is default for React Navigation's tab navigator), but it caused an issue where the tab navigator would take precedence over the stack navigator when the back button is pressed. This meant that inside a stack navigator if the back button is pressed, it would take the tab navigator to HomeTab instead of going back in the stack.

A better solution would be to move the settings screen to a different place that is not part of the main tab navigator. This also complies with material design guidelines which recommends back button not being used for navigation for a bottom tab navigator.

[jainkuniya]

This is a kinda complex issue, few months back I tried to fix it via https://github.com/zulip/zulip-mobile/pull/2135/files, which resulted in too complicated solution.

I thinks we should wait for #2702 (Upgrade to React Navigation v2), before reworking on #2135.