Fix TaskPositionerTests

In [1], we use getBounds() instead of getDimBounds(), which makes
original mock useless. This failure was not found since we don't run it
in presubmit. The first attempt is to try to use getBounds() instead of
getDimBounds(). However, it makes tests flaky since we may mock Task
in WindowTestBase#tearDown at the smae time, which lead to
MockException. We use real Task and setBounds directly to prevent the
flaky.
We also fixed some coding conventions in TaskPositionerTests

[1]: Id81d0b0a5f82be28fabed3ad22e713fc4fa7536d

Fix: 128820963
Test: atest TaskPositionerTests

Change-Id: Iaa50b74ccec19a399cfbfa45afa8136acb466055