Clean-up the PeopleActivity Compose code

This CL cleans up the Compose code of the PeopleActivity to align it
with best practices, in this case mostly making sure that all
Composables have an optional modifier parameter applied to the root
node emitted by the Composable.

Bug: 382448604
Test: Manual, checked that the UI is exactly the same as before.
Flag: EXEMPT trivial clean-up
Change-Id: I50d162f267fd50311981fe3836ad526145552dae