Introduce @Before / @After to ActivityTestJunit4

These annotations run in sequence from the highest superclass to the
implementation [1]. So ActivityTestJunit4 will run its @Before first
then the actual test class will run their own @Before method. This
means we can omit the explicit super.setUp implementations throughout
and rely on the annotations directly.

[1] https://junit.org/junit4/javadoc/latest/org/junit/Before.html

Bug: 383932124
Test: atest DocumentsUIGoogleTests
Flag: EXEMPT test change
Change-Id: I526d7873b6f76b54254d61e28c9ff627223776d5