Merge streamValidateAndCommit() and streamAndValidateLocked() (2/n)

Also move mStagingManager.checkNonOverlappingWithStagedSessions() outside
the lock. It is safe to do so since it accesses members which are immutable
after calls to validate[Apex|Apk]InstallLocked().

Now all calls to StagingManager are done outside the lock.

Bug: 161765186
Test: atest StagedInstallTest AtomicInstallTest StagedInstallInternalTest
Change-Id: I0d7cbe83a117a2d01eedc089a34778739d5fee36