This adjusts the public API documentation to no longer say that
digest/padding NONE means any digest/padding. This also changes the
implementation of legacy key generation and import to explicitly
list which digests/paddings the generated/imported key is authorized
for. Previously, such keys were simply authorized for digest NONE and
padding NONE.
Bug: 22556114
Change-Id: Id02d9450a07de16ccb795b76b6de0006dd49dcca
e4928a2912