Prevent findSession from creating new sessions

findSession in the SessionLibrary wasn't detecting
the case of a sessionId not found.  Since
DefaultKeyedVector was used, a new session id would
get created instead of returning a null sp.

Test: drm hidl hal vts test

bug:36006836
Change-Id: Iff00d8b84188d2bf9f26bceccf645bb730076378