init selinux.cpp: use a better way to detect if we run in Microdroid

We are now conditionally compiling init binaries & libinit for
Microdroid (adding -DMICRODROID=1 cflag), so instead of checking for the
presence of the /system/etc/selinux/microdroid_precompiled_sepolicy we
can check if the code is compiled for Microdroid.

In a follow-up changes we can split the sepolicy loading logic into 2
separate headers (one for Android and one for Microdroid) and include
the necessary one depending on the target we compile for.

Bug: 287206497
Test: atest MicrodroidTestApp
Change-Id: Id9c837d03a96ff9564688d33955ec85094eee487