Remove SystemState dependency from HttpFetcher and InstallPlan.

The SystemState class is an aggregation of all the update_engine
singletons, making it easy to handle cross-dependencies between these
singletons. Nevertheless, since we split the code into a smaller
libpayload_consumer library we need to remove the global dependencies
on the SystemState class from this library and specialize those
dependencies to the actual required class.

Bug: 25773375
TEST=FEATURES=test emerge-link update_engine; mma

Change-Id: I8800157c969db6a8d168f33ac2c6aad4f34fa236
diff --git a/payload_consumer/install_plan.cc b/payload_consumer/install_plan.cc
index f404c20..a2a1b7b 100644
--- a/payload_consumer/install_plan.cc
+++ b/payload_consumer/install_plan.cc
@@ -89,18 +89,18 @@
                 powerwash_required);
 }
 
-bool InstallPlan::LoadPartitionsFromSlots(SystemState* system_state) {
+bool InstallPlan::LoadPartitionsFromSlots(BootControlInterface* boot_control) {
   bool result = true;
   for (Partition& partition : partitions) {
     if (source_slot != BootControlInterface::kInvalidSlot) {
-      result = system_state->boot_control()->GetPartitionDevice(
+      result = boot_control->GetPartitionDevice(
           partition.name, source_slot, &partition.source_path) && result;
     } else {
       partition.source_path.clear();
     }
 
     if (target_slot != BootControlInterface::kInvalidSlot) {
-      result = system_state->boot_control()->GetPartitionDevice(
+      result = boot_control->GetPartitionDevice(
           partition.name, target_slot, &partition.target_path) && result;
     } else {
       partition.target_path.clear();