Generate linkerconfig per mount namespaces

There are two namespaces from init - bootstrap and default - and those
will have different set of APEX modules. To support difference between
two namespaces, linker config should be generated per namespace and each
namespace should use its own linker configuration. As a first step of
the work, this change will create different mount point for each
namespace, and re-generate linker config after APEX mount from each
namespaces.

Bug: 144664390
Test: m -j passed & tested from cuttlefish
Change-Id: Iac2e222376ec4b0ced6c29eed18b21d39ff0b1ba