Fix flaky of DocumentLoaderTest by mock context

DocumentLoaderTest is impacted by the whole system stability. To
make the original test to be DocumentLoaderPerftest as LargeTest.
The default atest DocumentsUITests run DocumentLoaderPerftest.

To fix the flaky test by using mock Context, limit the context only
return the mocked ContentResolver that return InspectorProvider's
instance.

Fixes: 120431745
Bug: 121002105
Test: atest \
  DocumentsUITests:com.android.documentsui.inspector.DocumentLoaderTest
Test: adb shell am instrument -w -e class \
  com.android.documentsui.inspector.DocumentLoaderPerfTest,\
  com.android.documentsui.inspector.DocumentLoaderTest \
  com.android.documentsui.tests/androidx.test.runner.AndroidJUnitRunner

Test: adb shell am instrument -w \
  -e notAnnotation androidx.test.filters.LargeTest \
  -e class com.android.documentsui.inspector.DocumentLoaderPerfTest,\
  com.android.documentsui.inspector.DocumentLoaderTest \
  com.android.documentsui.tests/androidx.test.runner.AndroidJUnitRunner

Change-Id: I5cb0c1f98d96d81b22a26b00c7df67bc15334cf5