feat(make): add install-android-app target and default help listing

- add `help` target and make it the default goal (bare `make` lists targets)
- add `install-android-app`: build debug APK and install on the selected device
- device selection via SERIAL/$ANDROID_SERIAL; several devices list and stop
- document the target in android/README.md

android/README.md

@@ -29,8 +29,21 @@ From the repository root:
 
 ```sh
 make android                        # runs :app:assembleDebug (ANDROID_SDK=... to override)
+make install-android-app            # build and install the debug APK on a device
 ```
 
+`install-android-app` builds the debug APK and installs it onto the sole connected
+device. To choose a device explicitly, pass its serial number:
+
+```sh
+make install-android-app SERIAL=emulator-5554
+```
+
+`SERIAL` also falls back to `$ANDROID_SERIAL`. When several devices are connected and
+`SERIAL` is not set, the target prints the list of devices (only those in the `device`
+state) and stops without installing. `ADB=...` overrides the `adb` binary (defaults to
+`$(ANDROID_SDK)/platform-tools/adb`).
+
 Debug output: `android/app/build/outputs/apk/debug/app-debug.apk`.
 
 ## Signing (release)