Revise secure key import API after review

• WrappedKeyEntry: add doc (based on IKeymasterDevice.hal comments)
• StrongBoxUnavailableException: add public ctors, match superclass
• SecureKeyImportUnavailableException: new first class exception
• ORIGIN_SECURELY_IMPORTED: elaborated on properties

Test: make doc & review output
Bug: b/74218267
Change-Id: Ice9adc60ede618870e57bb58ca66fd0218cd2bf7