Connected display: TestHandler is top-level class

This is needed to test an upcoming DisplayBlock unit test which will
not be an ExternalDisplayTestBase child, so won't get a TestHandler
automatically.

Test: ExternalDisplayPreferenceFragmentTest
Bug: b/352648432
Flag: EXEMPT refactor
Change-Id: I595f647caa513d5baa8bd214778fc41ca38de928
2 files changed