Don't create vendor_init in microdroid

vendor_init is a subcontext of init which is responsible for handling
the vendor-defined services and vendor-defined actions. This is mainly
to enforce the Treble architecture in which the vendor components are
provided with a less-privileged context than the system components
because they are less-trusted.

However, in case of microdroid, both the system and the vendor
partitions are from the same entity. As VMs don't have direct access to
any of the underlying hardware, the vendor partition is targeting the
virtual platform, not the real hw platform. There really is no need for
the discrimination.

This CL disables the creation of the vendor_init subcontext when init
runs in microdroid.

Bug: 201363575
Test: atest MicrodroidHostTestCases
Change-Id: Ie5e47d84e9e245565239b4f2159e8182b457699d