Always enable hardware layers in widgets in keyguard while dragging.

This fixes a performance issue on the 7" tablet where disabling hardware
layers in landscape mode resulted in extremely bad drawing performance.

The code now enables hardware layers for all the widgets while the user
is dragging.

Fixes bug 7274971

Change-Id: Iafd18fb317fcd3caad45791fe5a0f111d40a1ed4