TouchpadDebugView tests: Use TestableContext as a @Rule

This is the way that TestableContext's JavaDoc recommends, to let it
clean up after itself once the test finishes, so we should do it anyway.
However, unfortunately, it doesn't fix b/339924248, where
com.android.test.input.AnrTest fails if it's run after the touchpad
debug view tests.

Bug: 339924248
Test: $ atest \
    InputTests:com.android.server.input.debug.TouchpadDebugViewTest \
    InputTests:com.android.server.input.debug.TouchpadDebugViewControllerTests
Flag: TEST_ONLY
Change-Id: Ia5a480ccbdc05491c7e0bab8c269aa5bd55a205c