docs: document adb firmware sideloading on Android

The companion app takes an adb broadcast that flashes a pbz without
touching the phone UI (coredevices/mobileapp PR 321). Document it next
to the existing manual sideload steps.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Adrian Pascu <adrian@pascu.be>

Author: 2 people

docs/development/building_fw.md

@@ -47,3 +47,21 @@ onto your sealed watch:
 The resulting `.pbz` file will be located in the `build/` directory. Transfer this file
 to the device paired to your watch, then, in the Pebble app, enable `Settings -> Show debug options`.
 Go back to the Devices tab, tap your watch, then `Firmware Update Debug -> Sideload FW`, and select the `.pbz` file.
+
+On Android, flashing repeatedly is better scripted over adb, which installs without asking. This route
+is open to an adb shell, which holds the `android.permission.DUMP` the broadcast requires,
+and it needs `Show debug options` on in the app. `$PBZ` is the file from `build/`:
+
+```shell
+adb push $PBZ /data/local/tmp/firmware.pbz
+adb shell am broadcast -a coredevices.pebble.SIDELOAD_FIRMWARE \
+    -n coredevices.coreapp/coredevices.pebble.firmware.FirmwareSideloadReceiver \
+    --es path /data/local/tmp/firmware.pbz
+```
+
+The app waits up to a minute for the watch to be connected, then starts the update and
+shows its progress on the watch card in the Devices tab.
+
+Not every Android version lets an app read `/data/local/tmp`. If the app reports the file
+as missing, create `/sdcard/Android/data/coredevices.coreapp/cache` and push it there
+instead.