Keep previous implementation as "transactions (legacy)"
The proxy is not yet able to generate traces because no
adb command is available at the moment.
Known issues:
- Enum fields are not translated into values in Winscope
(only IntDef support)
Prepare the codebase for the new SF transaction tracing
Bug: 167521734
Test: run winscope and collect a transactions trace
Change-Id: Ie489ee1a63056377c1699e14df8de975c2978085