Let #startPreRebootVerification take a session object (1/n)

Most callers can pass a session object directly. We don't need to
call getStagedSession() to remap an id to a session object.

Bug: 167996901
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: If5af54bc15b98a085f55fa4669da437ab9d2506e
1 file changed