Fail committing new staged session if another one is already in progress

This ensures that there might be at most one committed non-finalized
(i.e. either applied or failed) session.

Note that from StagingManager POV atomic staged install is considered to
be a single session that is represented by the parent session of an
install.

Test: run adb install --staged foo.apk twice.
Test: StagedRollbackTest
Bug: 127296534
Change-Id: I497723173745d4f6eb64b7d22fa3adb0f9e8f6c0