Zeroize internally-retrieved unified profile passwords

Unlike normal LSKFs, unified profile passwords are often retrieved
internally by LockSettingsService instead of being passed into
LockSettingsService's Binder interface.  Make sure that any
internally-retrieved unified profile passwords are explicitly zeroized
when they are done being used, since otherwise they may remain in memory
afterwards where they may be found in a ramdump.

Test: atest FrameworksServicesTests:com.android.server.locksettings
Bug: 320392352
Bug: 416768837
Flag: EXEMPT bugfix
Change-Id: I794d6e25d9fbb60c116cafc436033ad35697f96a