bootstrap bionic are moved to ./bootstrap subdir

Bootstrap bionic (the bionic in the system partition that is for the
early processes before the runtime APEX is ready) is now located under
./bootstrap subdirectory. i.e.,
* /system/lib[64]/bootstrap/lib{c|dl|m}.so

Before this change, the bootstrap bionic were self bind-mounted. This is
causing problem with `adb sync` because the path which is a mount point
couldn't be unlinked and thus rewriting is not done.

Now, the mount points and mount sources are clearly separated.

Bug: 120266448
Test: m libc.bootstrap libm.bootstrap libdl.bootstrap
Following files are found under system and recovery/root/system
./lib[64]/bootstrap/libc.so, libdl.so, libm.so

Change-Id: I90399b3181988814ce07875df8c1a36599f3337f