Zygote: do not fail if CE app data dir not found.

When app data isolation is enabled, Zygote assumes that the CE
directories are there. Indeed this is normally always the case; on first
boot, the directories for the pre-installed apps are created by the
PackageManager. For newly installed applications, PackageManager will
make sure the CE dirs are created.

However, it is possible for the device to get into a state where these
directories are deleted, even for pre-installed apps. In that case, the
device will now refuse to boot, because critical system apps like
SystemUI cannot fork from Zygote as its CE data dir is missing.

Change the failure to a warning, which will allow devices to boot in
such cases. This effectively means that for the current boot, app data
isolation for CE directories will not be active. The directories will
get created later, and subsequently app data isolation will work again
on the next boot.

Currently the only way we know how to get in this situation is through
a case where user data cannot be 'prepared', and we trash all user data.
In such a case, there are no 3rd party apps installed, and any apps that
will be installed in the boot cycle will have functioning app data
isolation from the start.

Bug: 305658663
Test: Manual

Change-Id: I04d701cf03352c23bc7fd6dc99f181cb1ed8d804