Remove null-checks (5/n)

1. Remove the check in handleMessage() since all callers of
   obtainMessage() pass non-null sessions.
2. Remove the check in startPreRebootVerification() since
   the passed-in session is non-null.

Bug: 167996901
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: I6c5b5a1357130d3c9f4889b7e6297faf7f2c20a4