[femu] remove using custom tag for aemu

Fuchsia's vdl recipe_module now always uses the same version of aemu
http://fxr/421174 used by fuchsia in-tree

Fail a test step if emulator fails to launch. I noticed this when the
tests started failing this morning due to http://fxb/59041 but the
builder didn't mark the fails (because run_vdl_test.sh successfully
shutdown the emulator that didn't start correctly). So now we require
that all exit codes collected from launch, kill, and test_run to return
0.

Bug: https://github.com/flutter/flutter/issues/64343
Change-Id: I04fa936df59e27b5d358a3698686ee5d246e5204
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6060
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Yuan Zhi <yuanzhi@google.com>