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/aosp/dynamic_partition_test_utils.h b/aosp/dynamic_partition_test_utils.h
index c7be1cb..c518382 100644
--- a/aosp/dynamic_partition_test_utils.h
+++ b/aosp/dynamic_partition_test_utils.h
@@ -47,7 +47,7 @@
 
 constexpr const uint32_t kMaxNumSlots = 2;
 constexpr const char* kSlotSuffixes[kMaxNumSlots] = {"_a", "_b"};
-constexpr const char* kFakeDevicePath = "/fake/dev/path/";
+constexpr std::string_view kFakeDevicePath = "/fake/dev/path/";
 constexpr const char* kFakeDmDevicePath = "/fake/dm/dev/path/";
 constexpr const uint32_t kFakeMetadataSize = 65536;
 constexpr const char* kDefaultGroup = "foo";
@@ -112,7 +112,7 @@
 }
 
 inline std::string GetDevice(const std::string& name) {
-  return kFakeDevicePath + name;
+  return std::string(kFakeDevicePath) + name;
 }
 
 inline std::string GetDmDevice(const std::string& name) {