Refactor the activity instrumentation test cases

```04-21 01:13:57.683 I/TestGrouping( 1863): TestCase class com.salesforce.androidsdk.util.BaseActivityInstrumentationTestCase is missing a public constructor with no parameters or a single String parameter - skipping```

This seems to be causing the problems with our test runs. The base class wasn't adding too much value anyway, since we don't have too many activities. This simplifies things and should keep Jenkins happier.