extensions: Minimally port system.display for Desktop Android

This change ports a usable subset of chrome.system.display on Desktop
Android. Extensions can now query display information with getInfo() and
subscribe to onDisplayChanged, with the API moved into the
Desktop-Android-enabled extensions-core schema and build wiring.

On Android, ChromeExtensionsAPIClient now creates a display provider via
DisplayInfoProviderAndroid. It fills in name and dpiX/dpiY from platform
display data and now guarantees positive DPI fallback even when display
metadata and scale factor are missing.

The CL adds SystemDisplayAPI keyed-service wiring for Android
extensions-core builds so onDisplayChanged listener lifecycle and
BrowserContext shutdown correctly start/stop display observation, and
declares the EventRouterFactory dependency required for the keyed API.
Android-only guard conditions are expressed with IS_ANDROID for
readability and consistency.

In extension browser tests, manifest conversion helpers are now
explicitly idempotent: if a manifest is already MV3 or already
service-worker based, the helper returns success without rewriting. This
keeps shared test-loading paths working for both legacy MV2 fixtures and
tests authored directly as MV3/service-worker extensions.

Bug: 417786011
Test: CQ
Change-Id: Icd9049677bf5da154467a2dd37beedf737ee0438
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7558132
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Di Wu <diwux@google.com>
Cr-Commit-Position: refs/heads/main@{#1586847}