[power][broker] Use assertive deps for unsatisfiable element

* Previously, when an element was removed, we replaced all orphaned
  dependencies on that element with an opportunistic dependency on a
  synthetic internal "unsatisfiable" element. Since this element never
  had any assertive dependencies, it would never be raised, so leases
  with orphaned dependencies would remain unsatisfied.
* Now that we are removing opportunistic dependencies, we need a
  replacement for this. We now instead use an assertive dependency but
  have the element ignore requests from power broker to raise its level,
  functionally making it impossible to satisfy dependencies on this
  element.

Fixes: b/474606784
Change-Id: I2a6d6130c4db03a76b6107771698b88aa150b7c4
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1470800
Reviewed-by: Onath Dillinger <claridge@google.com>
Fuchsia-Auto-Submit: Kyle Gong <kgong@google.com>
Commit-Queue: Kyle Gong <kgong@google.com>