Revert "Update TestableContext annotation from ClassRule to Rule"

This reverts commit b8355485.

Reason for revert: DroidMonitor: Potential culprit for http://b/419887567 - verifying through ABTD before revert submission. This is part of the standard investigation process, and does not mean your CL will be reverted.

Bug: 414928899
Bug: 419887567
Change-Id: I8e053add06a1fe9509f8c864138c011435c92840