Don't create separate mount namespaces for pre-apexd processes

It is causing problem to the sdcardfs. Specifically, re-mounting events
for /mnt/runtime/{runtime|write} done by the vold process (which is a
pre-apexd process) are not being propagated to other mount namespaces.
As a result, SDCard access isn't working.

The propagation problem is a bug in sdcardfs which is fixed by [1].
However, we can't make all Android devices to have the patch at the same
time. Therefore, by default the separate mount namespace is not created
and will be created only for the devices where the kernel patches are in and
ro.apex.bionic_updatable sysprop is set to true.

[1]
d73d07673edbdbe78e1a7d00e7827ba9bfd86a59 ("ANDROID: mnt: Fix next_descendent")
b5858221c1c4f4bdc9ef67eb75ecf22580368820 ("ANDROID: mnt: remount should propagate to slaves of slaves")

Bug: 122559956
Test: m
Test: device boots
Test: sdcard works (e.g. camera can take pictures)
Test: atest android.appsecurity.cts.ExternalStorageHostTest
Change-Id: I7a309bab46356ee5782f34c5963d1760963c0b14