Keyguard: remove duplicated code

Patchset 2: fixed whitespace error
Patchset 3: a few more tiny changes
Patchset 4: some code moving, removing an unnecessary member
Patchset 5: make toggleSilentMode() a bit easier to read
Patchset 6: revert most of the cleanup
Patchset 7: fix syntax error
Patchset 8: fixes pointed out by Danny Baumann

Change-Id: I4e5f8c92b223c1260891472dbf807d5ff53e5721