Add more enums for the client and process attributes of SystemApi

For the client attribute, we have
- MODULE_APPS for representing APKs implementing modules which are
considered part of the platform
- MODULE_LIBRARIES for representing jars inside modules.

For the process attribute, we have
- SYSTEM_SERVER for representing the system_server process.

For example, @SystemApi(client=MODULE_LIBRARIES,process=ALL) actually
means that the API is meant to be available for classes in the
bootclasspath but not to the classes from the app classloader.

Bug: 140202860
Test: m
Change-Id: Iba6a564a0967e4d109164849ee6d6500d46469de