TranformFstabForDsu adds missing partition entries to fstab

Fix odd bug that TranformFstabForDsu doesn't emplace entry to fstab.

.avb_keys should be set to "/avb", the directory that stores all DSU
avbpubkeys, instead of appending.

Bug: 165471299
Test: Boot a multipartition DSU package
Change-Id: I62ee2f8bf7113f6d2af8cc34ef19c9743029ea0a
diff --git a/fs_mgr/fs_mgr_fstab.cpp b/fs_mgr/fs_mgr_fstab.cpp
index 54102ec..233d15f 100644
--- a/fs_mgr/fs_mgr_fstab.cpp
+++ b/fs_mgr/fs_mgr_fstab.cpp
@@ -640,13 +640,14 @@
             entry.fs_mgr_flags.wait = true;
             entry.fs_mgr_flags.logical = true;
             entry.fs_mgr_flags.first_stage_mount = true;
+            fstab->emplace_back(entry);
         } else {
             // If the corresponding partition exists, transform all its Fstab
             // by pointing .blk_device to the DSU partition.
             for (auto&& entry : entries) {
                 entry->blk_device = partition;
                 // AVB keys for DSU should always be under kDsuKeysDir.
-                entry->avb_keys += kDsuKeysDir;
+                entry->avb_keys = kDsuKeysDir;
             }
             // Make sure the ext4 is included to support GSI.
             auto partition_ext4 =