Adapt test app layout to smaller screens

Wrap test activity button region in a scroll view so that
all items can be made visible on screen, even with smaller
devices.

Test: atest CtsAccessibilityServiceTestCases:AccessibilityEndToEndTest

Bug: 375516886
(cherry picked from https://partner-android-review.googlesource.com/q/commit:951f1115bb0e81519e48c2cb81144b18d5cfe186)
Flag: TEST_ONLY
Merged-In: Ib2ea6a030770ddea78bb759f1fa5365a8dff1f93
Change-Id: Ib2ea6a030770ddea78bb759f1fa5365a8dff1f93