Android: honor a port in the metro.host system property

[janicduplessis]

Summary:

On Android the metro.host system property overrides the dev server host, but the port is always the compiled react_native_dev_server_port resource. So there is no way to point an installed app at a Metro on another port from outside the app: the only runtime override is debug_http_host in the app's SharedPreferences, which is set from the dev menu, or by rewriting the prefs XML through run-as on a debuggable build. iOS has this covered: RCTBundleURLProvider takes RCT_jsLocation from user defaults and, when the value contains a colon, uses it verbatim as host:port (serverRootWithHostPort).

This makes metro.host behave the same way: a value containing a colon is returned as-is, otherwise the compiled port is appended as before. adb shell setprop metro.host 10.0.2.2:8083 then does on an emulator what defaults write <bundleId> RCT_jsLocation localhost:8083 does on a simulator.

The case where this matters is several checkouts of one app, each with its own Metro port (git worktrees, parallel agents), sharing one build: the app is compiled once with port 8081 and pointed at the right port at launch. That already works on iOS; Android needed the prefs rewrite.

An IPv6 literal in metro.host also contains colons, but that was already broken (::1:8081 is not a valid authority; it would need brackets), so no working configuration changes.

Changelog:

[ANDROID] [ADDED] - metro.host accepts a host:port value, matching RCT_jsLocation on iOS

Test Plan:

yarn format-check-kotlin passes.

RNTester built from this branch (:packages:rn-tester:android:app:assembleDebug), installed on an API 34 google_apis arm64 emulator (adb root is needed to set the property; a Play image refuses setprop, which is a pre-existing limitation of metro.host). Metro started with react-native start --port 8099; nothing listening on 8081.

metro.host	result
10.0.2.2:8099	app loads; Metro on 8099 logs BUNDLE js/RNTesterApp.android.js
10.0.2.2	unchanged: compiled port appended, app tries 10.0.2.2:8081 and fails to connect