Add shell command to list app functions in JSON

Also, execute-app-functions now output JSON

See https://bit.googleplex.com/#/tonymak/6386020876550144 for example output.

dumpsys app_function is for human to parse while this new command is for
program to process.

Bug: 427146112
Test: Added test and run the commands
FLAG: EXEMPT shell command only

Change-Id: I041fca7db0339021a7cece45a4700b383d42e9c6