[RCTTouchHandler] Add a small optimization to coordinate conversion

[Saadnajmi]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Back in #1862, we made a fix to make sure that macOS screen coordinates (where the Y coordinate is flipped) are converted before being sent to JS, where we assume the coordinate system that every other platform uses (AKA, origin at top left). @lenaic pointed out there's a simpler way to do this, so let's make a small optimization. To quote:

Using this line to assign rootViewLocation instead of the weird _cachedRootView.window.contentView conversion fixes the flipping, allowing to remove the flipping you added below.

Note, it seems we've changed this particular block before, with #1619 . Seems it's a strongly-hit code path 😄

Changeling

[MACOS] [CHANGED] - Add a small optimization to RCTTouchHandlers' coordinate conversion

Test Plan

Sanity checked the Pressable test page with and without the change and didn't really see a difference.