Rename partner-apex-allowlist to vendor-apex-allowlist

Also rename corresponding field and getter in SystemConfig.java, and add
new ALLOW_VENDOR_APEX flag to control what partitions can specify
allowed vendor APEXEs (so far: /product, /oem and /vendor).

Bug: 189274479
Test: atest FrameworksServicesTests:SystemConfigTest
Change-Id: I680090d7a1237cb95c1aa4ea3975c50d06b35fcc