[iOS] Fix issues setting Shell TabBar appearance

[jsuarezruiz]

Description of Change

iOS portion of the following PR
#15447

Priority of how the colors are applied.

The APIs available to us are fairly limiting in scope. Ideally, we would make this all work through the VSM and just use two APIs (IconColor/TitleColor) but for now we are just making these APIs consistent between the platforms.

The three properties users have to influence the TabItem coloring are: TabBarUnselectedColor, TabBarTitleColor, and TabBarForegroundColor

• If the user only sets Shell.TabBarTitleColor that will color the Title and Icon of the selected item. I realize this is a bit weird for TitleColor to change Icon color but this is currently how Android works and it's not really worth breaking this until we just introduce new APIs.
• If Shell.TabBarTitleColor is not set then the TitleColor will match the TabBarForegroundColor
• If TabBarForegroundColor is set and TabBarUnselectedColor isn't set then TabBarForegroundColor will be the color used for the text/icon of the selected TabItem
• If TabBarUnselectedColor is the only thing set then that will be used for the text/icon color on the unselected Tab Item.

Examples

• TabBarTitleColor: Green
  • Selected Icon is Green, Selected Text is Green, Unselected item matches system colors
• TabBarForegroundColor: Blue
• Selected Icon is Blue, Selected Text is Blue, Unselected item matches system colors
• TabBarForegroundColor: Blue, TabBarTitleColor: Green
• Selected Icon is Blue, Selected Text is Green, Unselected item matches system colors
• Shell.ForegroundColor: Blue, TabBarTitleColor: Green (ForegroundColor propagates to TabBarForegroundColor).
• Selected Icon is Blue, Selected Text is Green, Unselected item matches system colors
• TabBarForegroundColor: Blue, TabBarTitleColor: Green, TabBarUnselectedColor: Pink
• Selected Icon is Blue, Selected Text is Green, Unselected text and Icon is Pink

Breaking changes

• Shell.ForegroundColor and Shell.TabBarForeGroundColor will now affect the android tab colors. If a user has ForegroundColor set, then the tab will match according to that color.

Fixes (partially) #6718
Fixes #16551

[jfversluis]

Agreed that there is some weirdness in how the API works, but love the detailed explanation. Something we need to fix down the road with improved/new APIs.

Already pinged @davidbritch to maybe make things a bit more clearer through documentation.

Added a couple of nits to be more consistent with the use of != null vs is not null, otherwise I thinks looks good