init: no bootstrap mount namespace for microdroid

The reason for two mount namespace (bootstrap/default) is that we have
two sets of different APEXes to activate. But in microdroid, we don't
need two separate mount namespaces because there's only one set of
non-updatable APEXes in built-in directories and payloads (passed from
host Android).

Bug: 185991357
Test: MicrodroidHostTestCases
Change-Id: I4469f3b1d90f9543ecb980a450eaaf1c20b94c58