vold: add getUnlockedUsers() method to Binder interface

This is needed so that system_server can remind itself about which users
have their storage unlocked, if system_server is restarted due to a
userspace reboot (soft restart).

Bug: 146206679
Test: see I482ed8017f7bbc8f7d4fd5a2c0f58629317ce4ed
Change-Id: I02f0494d827094bd41bcfe5f63c24e204b728595
(cherry picked from commit 1799debfd6561ca7348880bb59ad8c059f4891b0)
5 files changed