Add a skeleton of installPackages AIDL function

A minimal possible change required to make non-staged APEX install
plumbing work. Actual implementation will be done in a follow up CL.

Test: adb install shim.apex
Test: atest CtsStagedInstallHostTestCases
Bug: 187864524
Change-Id: I9f0f9562f3ff1c567cb1e75c51235610ab1eab69
(cherry picked from commit 817effbd1ec3be5af2d143052e2e593969b1f9d1)
5 files changed