Revert fix for FingerprintManager leak

Bug: 38509779
Bug: 32295793

Setting to null causes problem seen in b/38509779
Using weakreference will cause problems if the application
declares AuthenticationClient as a weak reference as well and would
very likely break a lot of existing apps.

Test: manual
Change-Id: Iedbdd76f3a402b766ce8a799bf76324d1164a0ec