Parse missing mount option.

Unrecognized mount option was causing early finish, leaving seinfo
and many other arguments undefined.

Bug: 123319678
Test: manual
Change-Id: If69f4d18e57ffd826075c33c47ec9bdd31e3badf
(cherry picked from commit 5bf9d72d1291278b415f0734d20fb4cf5a54c58d)
(cherry picked from commit 36244ba6c4d1ee7ca0f39cb5a5ddd9b25a3ac480)
diff --git a/core/java/com/android/internal/os/ZygoteArguments.java b/core/java/com/android/internal/os/ZygoteArguments.java
index df89b26..24a08ca 100644
--- a/core/java/com/android/internal/os/ZygoteArguments.java
+++ b/core/java/com/android/internal/os/ZygoteArguments.java
@@ -339,6 +339,8 @@
                 mMountExternal = Zygote.MOUNT_EXTERNAL_FULL;
             }  else if (arg.equals("--mount-external-installer")) {
                 mMountExternal = Zygote.MOUNT_EXTERNAL_INSTALLER;
+            }  else if (arg.equals("--mount-external-legacy")) {
+                mMountExternal = Zygote.MOUNT_EXTERNAL_LEGACY;
             } else if (arg.equals("--query-abi-list")) {
                 mAbiListQuery = true;
             } else if (arg.equals("--get-pid")) {