Add KeyChain.KEY_ALIAS_SELECTION_DENIED constant.

Add KEY_ALIAS_SELECTION_DENIED contant to flag that no private key alias has
been chosen in onChoosePrivateKeyAlias, but no KeyChainActivity selection dialog
should be presented to the user.

Bug: 136649900
Test: run cts --test MixedManagedProfileOwnerTest#testDelegationCertSelection
Change-Id: I9aeea7be0c2a6172ca054f91d49183c843ecfa6e