Replace deprecated AndroidJUnit4.

AndroidJUnit4 from `androidx.test.runner` is deprecated, use
`androidx.test.ext.junit.runners.AndroidJUnit4` instead. See https://
developer.android.com/reference/androidx/test/runner/AndroidJUnit4 for
more details.

Bug: 354063239
Test: atest FrameworksCoreTests --\
 --module-arg FrameworksCoreTests:include-filter:android.net \
 --module-arg FrameworksCoreTests:instrumentation-arg:annotation:=android.platform.test.annotations.Presubmit \
 --module-arg FrameworksCoreTests:instrumentation-arg:notAnnotation:=android.support.test.filters.FlakyTest,androidx.test.filters.FlakyTest
Flag: TEST_ONLY
Change-Id: Idf0d91782ffcef28495a8875176b5a9bdfaea0f5