Theme work lock activity with the task primaryColor

This involves adding another system RPC, getTaskDescription(taskId)
gated on MANAGE_ACTIVITY_STACKS permission.

Bug: 31001762
Test: runtest -x frameworks/base/packages/SystemUI/tests/src/com/android/systemui/keyguard/WorkLockActivityTest.java
Change-Id: Ieb996f7fab5bc79737df570e35733551118118d3