Don't set mDestroyed for a staged session

This CL fixes the race condition in b/161647423#comment1. No matter
setStagedSessionFailed() happens before or after destroyInternal(), it
will not cause a staged session to disappear (by setting mDestroyed).

In line with b/161647423#comment8, we want to keep staged sessions
until they
1. get expired
2. abandoned before reaching the terminal state

Bug: 161647423
Test: atest StagedInstallTest StagedInstallInternalTest
Change-Id: I41b7030451086602c8439b50e4362a36007a49cb