Make the PeopleSpaceActivity screenshot testable

This CL makes the PeopleSpaceActivity screenshot testable by extracting
a ViewModel and ViewBinder out of it. See ag/19289788 for the associated
screenshot tests.

Note that I tried to change the code inflating and updating the View as
less as possible, to avoid introducing bugs. Once this CL and the
associated screenshots are submitted, I will go ahead and refactor this
code even more. This CL is meant to be an example of the kind of
refactoring required to make a UI screenshot testable, so I tried to not
make it too big.

Bug: 238993727
Test: atest PeopleSpaceScreenshotTest
Change-Id: Ib792bd5da41c9e8bdab6cba7108a249bab10ebd2