Small refactoring in first_stage_mount

The existing approach in first_stage_init/first_stage_mount makes it
harder to add conditional logic that should only be applied for
Microdroid. Additionally, it forces the FirstStageMount object to be
created twice.

This change refactors the control flow to make first_stage_init take the
ownership of the FirstStageMount object. It will help with the follow up
change (which will add logic to conditionally mount /vendor partition
while booting Microdroid). As a nice side effect, this refactoring also
fixes the problem of the FirstStageMount being created twice.

This change also merges the FirstStageMount and FirstStageMountVBootV2
in a single class, since nobody actually uses FirstStageMount.

Bug: 285855433
Test: device boots
Test: atest MicrodroidTestApp
Change-Id: I38a72c0f20e7c1ac70031498aeeca22b091fa827