Camera2: Refactor preview-based test case

- Rename the class to follow the convention: If a particular class X is being
tested, then the test class name should be XTest. This makes the test class
much less verbose.
- Extract the common code from all preview related tests, and form one base
class to avoid duplicate code.

Change-Id: Ia56432578d5cecfb277ee98e3b7c10606d6c99c8