liblp: Expose kDefaultGroup.

kDefaultGroup is the 'default' group that is never deleted
and has no size constraints. During a virtual a/b update,
the following series of action happens:
- move partitions out of groups that will be shrunk / deleted
  (into default)
- shrink / delete these groups
- grow / add other groups
- move those partitions to their destination group.

Otherwise, the metadata is inconsistent.

Bug: 138816109
Test: builds

Change-Id: I57cf4b44dda948377c6b3153756f469caa4652bc
diff --git a/fs_mgr/liblp/builder.cpp b/fs_mgr/liblp/builder.cpp
index dab3d55..247e3a9 100644
--- a/fs_mgr/liblp/builder.cpp
+++ b/fs_mgr/liblp/builder.cpp
@@ -19,7 +19,6 @@
 #include <string.h>
 
 #include <algorithm>
-#include <string_view>
 
 #include <android-base/unique_fd.h>
 
@@ -31,8 +30,6 @@
 namespace android {
 namespace fs_mgr {
 
-static constexpr std::string_view kDefaultGroup = "default";
-
 bool LinearExtent::AddTo(LpMetadata* out) const {
     if (device_index_ >= out->block_devices.size()) {
         LERROR << "Extent references unknown block device.";
diff --git a/fs_mgr/liblp/include/liblp/builder.h b/fs_mgr/liblp/include/liblp/builder.h
index 1a26240..a2294ef 100644
--- a/fs_mgr/liblp/include/liblp/builder.h
+++ b/fs_mgr/liblp/include/liblp/builder.h
@@ -24,6 +24,7 @@
 #include <memory>
 #include <optional>
 #include <set>
+#include <string_view>
 
 #include "liblp.h"
 #include "partition_opener.h"
@@ -37,6 +38,9 @@
 static const uint32_t kDefaultPartitionAlignment = 1024 * 1024;
 static const uint32_t kDefaultBlockSize = 4096;
 
+// Name of the default group in a metadata.
+static constexpr std::string_view kDefaultGroup = "default";
+
 // Abstraction around dm-targets that can be encoded into logical partition tables.
 class Extent {
   public: