Optimize SystemConfig feature parsing

A number of system features are dependent on global system properties,
as opposed to the usual XML feature sources. Currently, these are read
and applied redundantly for every different XML permission source. This
is unnecessary.

Instead, simply read and apply these features once, after all other
permissions have been parsed. This saves ~10ms on a recent phone
and up to ~100ms on more resource-constrained devices (e.g., Wear).


Flag: EXEMPT refactor
Test: presubmit
Bug: 203143243
Change-Id: I76ba9626e9fb67a4822aacb438277f7fd708907d