docs: android: Add documentation for the touch_replay tool

Bug: 1415447
Change-Id: I41093fd600d5e45b53c2c7abdf1c9159b64418f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4264636
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107690}

tools/android/touch_replay/README.md

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+# Android Touch Replay Tool
+
+This tool allows to record and replay touch events on Android devices. The
+primary goal is for the replays to be consistent with each other as much as
+possible to allow for repeating touch gestures many times with statistical
+analysis to follow. Maintaining consistency with the original sequence of
+touches is only a secondary goal.
+
+Recording on one device and replaying on another is likely not going to work
+because of differences in the structure of touch events. The way the kernel
+drivers emit the events depends heavily on device type and OS version.
+
+## How to use
+
+Please use an optimized release build to minimize potential delays between
+emitted events. Here is how to build it:
+
+```
+# autoninja -C out/AndroidRelease touch_replay
+```
+
+This produces an executable only runnable on the device. Push it to the device:
+
+```
+# adb push out/AndroidRelease/touch_replay /data/local/tmp
+```
+
+Before running the tool make sure it will be invoked with root privileges:
+
+```
+adb root
+```
+
+Run the tool. All interaction with the screen will be recorded until you press
+Ctrl+C.
+
+```
+# adb shell '/data/local/tmp/touch_replay record /data/local/tmp/touch_events.dump'
+```
+
+To replay change `record` with `replay` in the command above.
+
+The last piece of functionality of this tool is the help message:
+```
+# adb shell '/data/local/tmp/touch_replay --help'
+Usage: /data/local/tmp/touch_replay record|replay FILE
+
+Record input events to FILE or replay them from FILE.
+```
+
+### Usage with Perfetto
+
+It is important to be able to replay touch scenarios while the device is being
+traced. The `cpu_profile` tool is compatible with running the replay
+simultaneously. See about its usage in the [Perfetto
+documentation](https://perfetto.dev/docs/quickstart/callstack-sampling#prerequisites).
+
+Using Perfetto from the browser to record a trace on the device is less
+straightforward. While attached to the device, Perfetto may make the device
+invisible to the commandline `adb` tool:
+
+```
+adb: no device found
+```
+
+One workaround is to start replay and attach with Perfetto later:
+
+```
+adb shell 'nohup sh -c "sleep 15 && /data/local/tmp/touch_replay replay
+/data/local/tmp/touch_events.dump" >/dev/null 2>&1 &'
+```
+
+This will wait for 15 seconds before replaying. The adb command would return
+immediately. After that the tool runs on the device without depending on the
+existing adb session. This works by running the program as a daemon with the
+help of the `nohup` utility. Since both the sleeping and replaying needs to be
+daemonized, they are wrapped into a shell command together.
+
+Immediately after invoking the command above it should be possible to connect to
+the device with Perfetto (within the specified time interval, 15 seconds in this
+case) and start profiling.