Return installer mount mode for installers.

In the new FUSE world, we're interested to know the difference between
installers and apps that previously asked for WRITE_EXTERNAL. For
devices without FUSE enabled, they both map to the "write" sdcardfs
view, so for them this is a no-op.

Bug: 146189163
Test: device boots with and without FUSE
Change-Id: I634829eec8f0e7e65e4e740ce216dded23abc271

services/core/java/com/android/server/StorageManagerService.java

@@ -3812,7 +3812,7 @@ class StorageManagerService extends IStorageManager.Stub
                 }
             }
             if ((hasInstall || hasInstallOp) && hasWrite) {
-                return Zygote.MOUNT_EXTERNAL_WRITE;
+                return Zygote.MOUNT_EXTERNAL_INSTALLER;
             }
 
             // Otherwise we're willing to give out sandboxed or non-sandboxed if