Made keyguard pattern view unfocusable

Keyguard pattern view was receiving focus, causing a gray rectangular
background to show up when a physical keyboard was
connected, which isn't intended behavior as physical keyboards cannot
and should not interact with the pattern view.

Fixes: 328650320
Test: Manual - set lock screen auth method to pattern, wake up tablet
with spacebar/enter, ensure that there is no gray rectangle over the
pattern view.
Flag: NA
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f51cea53b19d7d10f8b574324b2cc077b7eaeba0)
Merged-In: I768174728eeb269b04d927c25535d586b9a2a6d8
Change-Id: I768174728eeb269b04d927c25535d586b9a2a6d8