Use a regular set for started users.

We want started users to be an ordered set; eg user 0 should always go
first. This is because volumes for users other than 0 depend on user 0
coming up first, because the volume for user 0 is the one mounting
sdcardfs.

Bug: 13789012
Test: atest AdoptableHostTest
Change-Id: Ic9119f0a24bd261e5362019836ac240b90c681c0
1 file changed