Use vendor/etc/ueventd.rc on microdroid

... because vendors should be able to extend ueventd.rc for their own
devices (in microdroid) and microdroid's system.img can't be amended.

Bug: N/A
Test: boot microdroid
Change-Id: I7ceaf3f87453c8e29f9d9bf03d4d1866185d4fec