fix(android): use the context's display for display metrics, not the device default

[DouweBos]

Summary:

DisplayMetricsHolder.initDisplayMetrics populates screenDisplayMetrics by calling getRealMetrics on WindowManager.defaultDisplay. defaultDisplay is always the device's primary display, regardless of which display the activity is actually running on.

When an activity runs on a secondary display (Samsung DeX, desktop mode an external monitor, a freeform window, or an emulator virtual display via am start --display N) its density and dimensions differ from the primary's.

PixelUtil (toPixelFromDIP / toDIPFromPixel / toPixelFromSP / getDisplayMetricDensity) reads screenDisplayMetrics for every dp <-> px conversion the layout engine performs, so Fabric lays out content at the primary display's scale into a window that doesn't have that scale. The visible content is clipped to a fraction of the activity window with the rest left black, and text renders at sub-pixel positions (blurry).

This is observable from JS — Dimensions.get('window') and useWindowDimensions() are correct, but Dimensions.get('screen') reports the primary display's scale:

window: { width: 1600, height: 720, scale: 1.5 }   <- the activity's display
screen: { width: 800,  height: 360, scale: 3   }   <- the device's primary display

This PR changes initDisplayMetrics to read from the display the Context is associated with, Context.getDisplay() on API 30+, falling back to WindowManager.defaultDisplay on older API levels, instead of always the device default. It also passes the view's own context (not the application context) from ReactRootView, so that context is associated with the activity's display.

Single-display behavior is unchanged: when the activity is on the primary display, the context's display is the default display, so the metrics are identical.

Fixes #56894 (also tracked in #55659).

Changelog:

[ANDROID] [FIXED] - Display metrics now reflect the activity's display instead of the device's default display, fixing layout scaling on secondary displays / desktop mode / freeform windows.

Test Plan:

Reproducer (standalone minimal app + commands): https://github.com/DouweBos/rn-secondary-display-repro

Unit tests

./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest --tests "com.facebook.react.uimanager.DisplayMetricsHolderTest" --tests "com.facebook.react.uimanager.PixelUtilTest"

A/B on rn-tester, built from source

Built rn-tester from this branch and from its parent commit (this commit reverted), installed both on the same Pixel 9 Pro AVD (API 36) with a virtual secondary display at a different density than the primary:

adb emu multidisplay add 1 2400 1080 240 0   # 2400x1080 @240dpi (1.5x); primary is 3.0x
adb shell am start -n com.facebook.react.uiapp/.RNTesterActivity --display 2
SF_ID=$(adb shell dumpsys SurfaceFlinger --display-id | awk '/Virtual display/ {print $2; exit}')
adb shell screencap -d "$SF_ID" -p /sdcard/d.png && adb pull /sdcard/d.png .

Before (parent commit) — screenDisplayMetrics taken from the primary display; the dp ↔ px conversion uses the primary's 3.0x density on a 1.5x display, so text is clipped/garbled ("Comp" instead of "Components", "Bu", "DrawerLay", …) and the bottom tab indicator is sized for the wrong width:

After (this branch) — metrics come from the activity's display; layout fills the window and all labels render fully and crisply:

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[DouweBos]

Sorry, incorrectly assumed the original PixUtil functions were still present. Updated the PR to a more stable fix by relying on the context and the associated display instead of the defaultDisplay it relies on right now.