Add timeout for mount on untrusted media

Under some circumstances it seems mounting a device can take a long
time. We already abort fsck if it runs for too long, so this is the last
remaining spot this could happen.

Bug: 339740354
Test: Mount a very slow device, observe no Android restart
Change-Id: I3607be269d3aeb4c2391d1d8e2372e3a6795e7c4
4 files changed