vold: Don't unmount /mnt/installer on start.

This is a slave bind mount of /mnt/user, created before we create the
two mount namespaces. Unmounting it here prevents us from re-creating it
in the right way, so leave it alone.

Bug: 134706060
Test: verify mount is still there after vold starts
Change-Id: Iaac91953cbb9abfef0aaac60f74b99b16c943f87
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index 98cb060..d8b1e32 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -806,7 +806,8 @@
 #ifdef __ANDROID_DEBUGGABLE__
              !StartsWith(test, "/mnt/scratch") &&
 #endif
-             !StartsWith(test, "/mnt/vendor") && !StartsWith(test, "/mnt/product")) ||
+             !StartsWith(test, "/mnt/vendor") && !StartsWith(test, "/mnt/product") &&
+             !StartsWith(test, "/mnt/installer")) ||
             StartsWith(test, "/storage/")) {
             toUnmount.push_front(test);
         }