Add IVold::destroyDsuMetadataKey()

destroyDsuMetadataKey() / destroy_dsu_metadata_key() calls
android::gsi::GetDsuMetadataKeyDir() to query the DSU metadata
encryption key dir and destroy the key.
This releases the resource and allows consecutive DSU installations to
use the same key *directory*, but not the same key *blob*.

Bug: 168571434
Test: 1. Install a DSU system.
  2. Boot the DSU system and reboot back to the host system.
  3. Wipe the DSU installation.
  4. DSU metadata key dir /metadata/vold/metadata_encryption/dsu/dsu is
     destroyed.
Change-Id: Ib851177315a5a266807f46ccfd446de1848232cf
diff --git a/Android.bp b/Android.bp
index 4852fe4..1c0ed17 100644
--- a/Android.bp
+++ b/Android.bp
@@ -54,6 +54,7 @@
         "libdiskconfig",
         "libext4_utils",
         "libf2fs_sparseblock",
+        "libgsi",
         "libhardware",
         "libhardware_legacy",
         "libincfs",