AppHost::_updateTheme is called 8x during launch

### Windows Terminal version

42e8de3

### Actual Behavior

This primarily happens due to `TerminalPage::_updateThemeColors` being called just as often. Once during `TerminalPage::Create`, but then again during `TerminalPage::WindowActivated` and `TerminalPage::_UpdatedSelectedTab` and then multiple times because the "BackgroundBrush" property changed, even though the actual color stayed constant during the entire time.
We should add one more more early returns in our event handlers, as far up the call chain as possible.

Trace AppHost::_updateTheme and TerminalPage::_updateThemeColors through TerminalPage::Create, WindowActivated, and _UpdatedSelectedTab, focusing on the BackgroundBrush change notifications during launch. Confirm where repeated calls can be skipped without changing the final theme, then verify that launch produces fewer updates while retaining the correct appearance.

Written by the indexing model from the issue text.