API feedback with better documentation.

+revert the Incremental installation API to be SystemApi as the corresponding native API is not exposed in the NDK.

Bug: 184713911
Fixes: 184713911
Test: atest PackageManagerShellCommandTest PackageManagerShellCommandIncrementalTest IncrementalServiceTest PackageManagerServiceTest ChecksumsTest
Change-Id: I466eb6d2f98aed75c22b92877d5e8cb3ddf839bc