Consolidate Keystore alias prefixes.

Currently, the keystore SPI assigns different prefixes to user key
entries depending on the algorithm. Symmetric keys (secret keys) get
the prefix USERSKEY_ and asymmetric keys (private keys) get the
prefix USERPKEY_. This distinction is superfluous, as the information
can always be retrieved from the key characteristics. Also moving
forward it is desirable to be able to import keys the nature
of which is not known a priori. In these cases the prefix cannot be
chosen meaningfully.

This patch deprecates one of the prefixes (i.e. USERSKEY_) and uses
the other for both types of keys. Legacy keys with the old prefix
can still be used, but all new keys will have the prefix USERPKEY_.

Bug: 63931634
Test: CTS test and Manual upgrade test with KeyStoreTool app
      Also performed upgrade test with device PIN set
Change-Id: I5b4bb0b0d2b82c276659d55b862150326bb68d5d