IThermalManagerTest: fix double-owned

The test object is owned by gtest and therefore also can't
be a callback (inheriting from RefBase which implies
ownership).

This bug is causing an abort now. To detect it automatically in the
type system, you can use the cflag
-DANDROID_UTILS_REF_BASE_DISABLE_IMPLICIT_CONSTRUCTION .

Fixes: 240371998
Test: atest libpowermanager_test
Change-Id: I68a2403d33cdbba225aee95a5dd22a96371904ce