Reduce demo user FUSE volume mount timeout

A long timeout seems to break demo users when calling
DevicePolicyManagerService#enableSystemApp.

We soft-revert Ia8f6f121448f6b5e484fd3e8cca845fdd89afc23 just for demo
users while investigating the root cause.

Test: Manual
Bug: 161702661
Change-Id: I8ff153f2649e3a1d05f2e3e6e382bbba8c79630c