InputMapperUnitTest: extract device creation from SetUp

When device creation occurs, configuration properties are copied from
the property map returned by EventHub::getConfiguration into the device
object. For this reason, some tests (such as the CursorInputMapper tests
that have yet to be migrated to InputMapperUnitTest) need to run some
setup code before the device is created. Other tests can simply call
createDevice immediately after InputMapperUnitTest::SetUp in their own
SetUp methods.

Bug: 283812079
Test: atest inputflinger_tests
Change-Id: I3ae3d407fb14e8f8ff061ec617003f423bce3f5e