[Contextual Edu] Fix repository unit test

The TemporaryFolder test rule is unable to create data file when running test, throwing:
java.nio.file.NoSuchFileException: /data/user/0/com.android.systemui.tests/cache/junit5103703972173410145
This CL removes the TemporaryFolder rule and adds "clear" method in repository for cleaning up the datastore after test, just like TutorialSchedulerRepository.

Bug: 384693910
Test: ContextualEducationRepositoryTest
Flag: com.android.systemui.keyboard_touchpad_contextual_education
Change-Id: I72a08ed98ff2055c07cbae39a097a32e43aaa9cd