Introduce microdroid variants of init_first_stage and init_second_stage

These variants will compile with -DMICRODROID flag, which will allow us
to exclude init features that are not needed for Microdroid, and
introduce features that only work in Microdroid.

Bug: 287206497
Test: build com.android.virt APEX
Change-Id: Ib9af0cfcdf06c70fc39e6e6ac8ef07bb69982969