Add rule to press home before ImeOpenCloseStressTest and AutoShowTest

Add PressHomeBeforeTestRule to press home before tests, which avoids
test failures when notification page is shown before tests.

Bug: 261775358
Test: atest com.android.inputmethod.stresstest.AutoShowTest
Test: atest com.android.inputmethod.stresstest.ImeOpenCloseStressTest
Change-Id: I55d8f8423429c6a6410722daa613b92e8da20ff5