Ensure 36dps around keyguard bouncer

Simplified bouncer size calculation by setting a default
lateral margin and removing unused dimens.

We can expect the bouncer to grow up to the requested size,
or shrink to incorporate the margins.

Change-Id: I085bc4148aa956b2107ab2a4853e8dd9fcbdce06
Fixes: 64560804
Test: runtest -x packages/SystemUI/tests/src/com/android/keyguard/KeyguardSecurityContainerTest.java
Test: look at keyguard on multiple screen sizes