Expose Handler APIs as @TestApi

CtsOsTestCases cannot access MODULE_LIBRARIES APIs. Thus these methods
need to be exposed as @TestApis for CTS access

Bug: 383405068
Test: atest CtsOsTestCases:HandlerTest
Flag: android.os.mainline_vcn_platform_api
Change-Id: If617d41b090320888150b84dbb377b3dec0f722a