Reset taskbarModePreference on tearDown()

This CL forcefully resets taskbarModePreference on tearDown() to ensure
the following tests are not affected by the changes made by
`testTaskbarPinningChangeInLockedTaskbarChange`.

Bug: 368322666
Flag: EXEMPT bug fix
Test: nexus_unit_test_multi_device_platform
Change-Id: I8bd8e8ca8b4b832ed17e86fbdfc9bda7bf9ab16e