init: skip SnapshotManager steps for microdroid variant

`IsMicrodroid()` is a compile time constant, so this allows all of the
SnapshotManager code to be elimated at compile time, which transitively
includes some big libraries, like zstd, lz4, and brotli.

When checked with the following command, the binary size goes from
3,299,754 to 1,510,044 (-55%).

size -A out/soong/.intermediates/system/core/init/init_second_stage.microdroid/android_arm64_armv8-2a_cortex-a55/init

Test: launch microdroid
Flag: EXEMPT ...
Change-Id: Id747ac37de80e23df9b7939f9bb82d0237beb4fa