Allow staging multiple sessions with non-overlapping packages (apk+apex)

CL contains changes made in PackageManager world to allow staging multiple
apexes on apexd.

Bug: 141843321
Test: atest StagedInstallTest#testInstallMultipleStagedSession_ApexOnly
Test: atest StagedInstallTest#testInstallMultipleStagedSession_PartialAbandon_ApexOnly
Change-Id: Ic7fbdddf30047cafb712a102556b70c6dab271b1
Merged-In: Ic7fbdddf30047cafb712a102556b70c6dab271b1
(cherry picked from commit da4eeabf)