Fixes in KeyguardManager doc.

he → the
remove unused import
keyboard → keyguard

(It is likely that when the API was originally documented, the keyguard
primarily locked interaction with a keyboard and this was not a typo.
In the present, Keyguard is an established concept that primarily
gates interaction with applications rather than input devices.)

Test: comments and lint only

Change-Id: I55d58baee02d9131f8c61e23935cd5010ff27f07