Rename "user key" methods in vold

Rename methods that refer to "user key" to be more precise about what
they mean.  For more details, see the corresponding frameworks/base
changes (I202ebbfd2b4f79fedb3ed120a8ad81500c126894 and
I5894beb97823dced5954e405d779fada49c79e8d).

No change in behavior except for some changed log messages.

Flag: exempt, mechanical refactoring only
Test: presubmit
Change-Id: I9edcb557172395f4f6cf8e837efcc06fcfefb37d
5 files changed