Formally define allowable IpcDataCache modules

Bug: 219609105

The allowable IpcDataCache modules are formally defined as a
@StringDef.  There is no change to the system APIs.

A future change may subdivide the SYSTEM module into small,
feature-specific modules, such as package manager and user manager.

Test:
 * atest FrameworksCoreTests:IpcDataCacheTest
 * atest CtsOsTestCases:IpcDataCacheTest

Change-Id: I15e358723a36b76c384ccfb70bed63e80d57c4c7