Handle adb sync with Bionic under /bionic

Background:
We now have two sets of Bionic: the bootstrap Bionic which is at
/system/{lib|bin}/bootstrap for early processes and the default Bionic
which is from the runtime APEX for all the others. In order to give the
same path for Bionic to both categories of processes, the init prepares
two mount namespaces and bind-mount appropriate Bionic files onto the
common mount points under /bionic. For example,
/system/bin/bootstrap/linker is bind-mounted to /bionic/bin/linker for
the early processes. Likewise, /apex/com.android.runtime/bin/linker is
bind-mounted to the same path for rest of the processes.

In addition, in order not to propagate mount events in one mount
namespace to the other namespace, /bionic itself is created as a mount
namespace (via self bind-mount) and its propagation type is set to
private.

Changes required:
This however requires some adjustments to adb sync and remount
mechanism.

For remounting, /bionic path should also be re-mounted for RW, because
it is a RO mount in the beginning. This remounting is done only for the
system-as-root devices where entire / can be re-mounted as RW.

For synching, the sync thread creates a temporary mount namespace where
there is no bind-mount. This ensures that a path that the thread handles
is pointing to the correct file that is expected from the client side.

In addition, push operation to /bionic path is done without unlinking.
This is required because the mount points under /bionic are gone in the
current mount namespace but are still active in other mount namespaces.
If unlinked, the existing mounts on the path are all silently removed.
In order to prevent the unwanted situation, the moint points are not
unlinked but truncated to 0. This however is not a significant problem
because the files that serve as mount points do not carry any
useful information (i.e. the content is meaningless).

Bug: 879416
Test: adb sync
adb push <random_file> /bionic/bin/linker64
adb push <random_file> /system/bin/bootstrap/bin/linker64
system/core/fs_mgr/tests/adb-remount-test.sh
Change-Id: Id87dc9ee7ec5c43d06b54969b55e2cb394329317