Do not map dynamic partitions on VABC devices

With VABC, we no longer need to map all partitions before
reading/writing, so don't try to map them.
1. modify GetPartitionDevice to return empty path for target partitions
on VABC
2. Add a separate GetMountableTargetDevice for obtaining a mountable
device path, specifically for postinstall

Test: treehugger
Change-Id: Ib1f608914fc49c677ce7389140ca79b028171191
diff --git a/common/boot_control_stub.cc b/common/boot_control_stub.cc
index 907f670..a1cc055 100644
--- a/common/boot_control_stub.cc
+++ b/common/boot_control_stub.cc
@@ -44,6 +44,15 @@
   return false;
 }
 
+std::optional<PartitionDevice> BootControlStub::GetPartitionDevice(
+    const std::string& partition_name,
+    uint32_t slot,
+    uint32_t current_slot,
+    bool not_in_payload) const {
+  LOG(ERROR) << __FUNCTION__ << " should never be called.";
+  return {};
+}
+
 bool BootControlStub::GetPartitionDevice(const string& partition_name,
                                          Slot slot,
                                          string* device) const {