Remove a.t.TestCase

Part of the work of removing JUnit and dependent android.test classes
from the Android API involves providing a static library that developers
can include in their test applications to ease migration. That library
will be built directly from the source (as opposed to android.jar which
is built from stubs) and so developers will be able to see classes and
methods that are not present in the stubs. This change is one of a
number of similar changes that cleanup the existing non-API code in
order to minimize the additional methods and classes exposed externally.
The basic approach is to remove unused classes and methods, use least
visible access modifier possible and generally minimize the amount of
publicly visible code.

The android.test.TestCase class is only referenced from the
DatabasePerformanceTests class. There do not seem to be any references
to that class either and there is a NewDatabasePerformanceTests.java
file in the same directory that looks to be a JUnit based replacement.

Bug: 30188076
Test: make checkbuild and ran FrameworkTestRunnerTests
Change-Id: Iafb26dc98a40715244f07d1c9e05ada5b1602ebf