Extract flags into their own Util class

The flags are all currently spread out across the codebase. This means
if we want to do any large scale changes based on them (e.g. change the
flag name OR union the flag with some other condition) it requires a
mass refactoring. Let's move the flags into their own class so this
becomes a much easier process.

To avoid additional usages getting added, create a presubmit check that
should error when new dependencies are added

Bug: 400475579
Flag: EXEMPT Moving flags into a helper class
Test: m DocumentsUIGoogle
Change-Id: If5c346141813afb68b24e1303362408c3cc9fd4a