Enable IpcDataCache test mode for CTS

The API IpcDataCache.setCacheTestMode() lost its @TestApi annotation
in an earlier commit.  This change restores that annotation so the
method can be used in CTS.

Flag: EXEMPT test-only
Bug: 396173886
Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
 * FrameworksCoreTests:IpcDataCacheTest
 * CtsOsTestCases:IpcDataCacheTest
Change-Id: Ic4026e862d11a654f05eae12b2819d1532188b81