Consolidate blur-disabled tests

- testNoBlurBehindWhenBlurDisabled was not testing what it claimed
  to test because it was never disabling blur

- testNoBackgroundBlurWhenBlurDisabled worked fine but did
  mostly the same things that the other test needs to do

Since these tests take 6 seconds to run, and do almost the same thing,
it's easier to combine them.

Change-Id: I82b6d319dfdaa66baa03b3ed61d381ec5cc9805e
Flag: EXEMPT test fix
Test: atest BlurTests
Bug: 380647664