AndroidTestCase.[set|get]TestContext are added as APIs

android.test.runner is using AndroidTestCase.[set|get]TestContext
methods from android.test.base. The problem is that the methods are
internal APIs and this is preventing us from hiding the impl library of
android.test.base to others.

Marking those two methods as APIs.

Bug: 157007292
Test: m
Test: android.test.runner

Change-Id: I8ea5421723a4a40d580956e5ceb51be70253f0eb