HSUM configuration in AOSP

* Moves HSUM configuration to AOSP (so it can be used by anyone, and can
  exist in generic systems)
* Defines a variable for selecting HSUM-variant products, that is, a way
  to select an alternate version of packages when required on HSUM
  devices.
* Utilises this mechanism to use an alternative version of
  TelephonyProvider (that has per-user databases) on HSUM devices.

Note that HSUM refers to Headless System User Mode, which is a
particular device mode in which user 0 isn't a human user.

Bug: 331960280
Bug: 347341495
Test: builds with correct configuration, with the appropriate version of
      TelephonyProvider
Ignore-AOSP-First: changes in topics with internal-first/internal-only projects
Change-Id: Ia8a1cc7dd43dbb488f1e23b7187416a528a59c5f