init: apexd is started in the current mount namespace

init starts services in "bootstrap" mount namespace until the "default"
mount namespace is ready even when init's current mount namespace is
"default".

apexd and linkerconfig are those processes to set up the mount
namespaces: apexd activates apexes and linkerconfig generates linker
configs.

Previously apexd is allowed to be started in the "current" namespace by
checking its "service name"(it should be "apexd"). But there can be a
certain environment apexd is started in a different way. For example, in
microdroid, apexd is started using "exec -- /system/bin/apexd --vm"
because it wants to run in a different execution mode.

So, instead of checking the service name, its executable's path is
checked against to allow apexd to be started in the current mount
namespace.

Bug: 179342589
Test: MicrodroidTestCase (microdroid boots)
Test: cuttlefish boots
Change-Id: I7c2490e15d481c28ddf382d2d3fdf58a78e467ec