Change storage size before starting test

Symptom:
CancelFromNotificationUiTest is failed because of broken pipe error.

Root cause:
super.setUp() method will change the storage size to 100MB.
On the other hand, this test case uses a 500MB file as a
dummy file. Therefore, the file capacity is insufficient
and an error occurs.

Solution:
After excuting super.setup(), reset the storage size again to 500MB.

Bug: 72137394
Test: Unittest of com.android.documentsui.tests
Change-Id: Ib9cc0211b4e8176ff5747b977a9dc485d439ea8d