Workaround to support low-area UDFPS on BP

1) Ensure the spacer measurement is clamped to 0. Negative values
   cause the layout to exhibit undefined behavior.
2) Instead of making the layout xml even more complicated, for low-area
   sensor devices, we just have onLayoutInternal() translate the icon
   and indicator to where it should be. Note that this can definitely
   cause overlap with the button bar, but at least this implementation
   allows the button bar to be shown (and thus pass curent CTS
   requirements). We can have additional refinement in the future.

Bug: 201510778
Test: atest CtsBiometricsTestCases
Change-Id: Ie9869f15fc3afddc3bd4392a2fd08efbf136cd6c