Don't send SESSION_COMMITTED broadcast for staged installs

Staged sessions currently send a SESSION_COMMITTED broadcast
when they are created, but not upon activation. Since there
is nobody active at activation to listen to the broadcast,
it should not be sent. This also prevents app shortcuts being
added to the launcher before a new version has been activated.

Bug: 139663018
Test: atest StagedInstallTest
Change-Id: Icf51b5611a1903eee58c3310bfb875eaf737c26f