Add Mock APIs for staged installs.

This CL just adds the isStaged setting on SessionParams, and if it is
set the session info are cached in a local variable. This allows
getStagedSession() (new APIs) to return the session information.

This should unblock upstream development while we implement the feature.

Test: adb install --staged, adb install-multi-package --staged. Wrote
small app to verify getStagedSessions returns correct info.
Bug: 118865310
Bug: 112669193
Bug: 120487127
Change-Id: I822be9e1737fe5cde6a6dd63621631cdef4f6fff