Use the new 'partition' field in 'ApexInfo' to identify vendor apexes

A new field 'partition' was added to `ApexInfo` recently which stores
pre-installed partition information as string (e.g. 'SYSTEM') in
aosp/3335753. Using 'partition' field for Subcontext vendor apex
initialization because the existing field `preinstalledModulePath` won't
be populated for brand-new apex (a new type we introduced recently).

Bug: 377111286

Test: atest CtsInitTestCases
Change-Id: I8970b3cb5884bdb949035f5bdc5b2e18618cc9cc