Check package overlaps for rebootless APEX (5/n)

Move the code to PackageSessionVerifier.

Note now the error is treated as a verification error instead of
a validation error. This will help debugging as the error message
is recorded.

Bug: 161121612
Test: atest StagedInstallInternalTest#testRebootlessUpdate_hasStagedSessionWithSameApex_fails

Change-Id: I8a206034cdf64a39c47ad105a4045a8098ad4386