input tracing: move backend code into common location

We'll need to use this code from the reader to implement lower-level
event tracing.

Test: take a trace using the adb shell perfetto command, open it in
      Winscope and check input events are present
Bug: 394861376
Flag: EXEMPT refactor

Change-Id: Ib9d22d8292198819877a09d30312828af635533e