Enable improved user separation by default.

Enable improved user separation by default.

This is already on for all Pixel devices with no problems observed.

If this causes issues with a specific device (e.g. vendor apps being
unable to access their data) it can be temporarily disabled by adding

PRODUCT_PROPERTY_OVERRIDES += ro.vold.level_from_user=0

to the device.mk file. Please file a bug if that happens.

Bug: 141677108
Test: presubmits
Change-Id: Ic9da534f1a5f4c9e3bd62ea5c09a3b11ebcb33e7
Merged-In: Ic9da534f1a5f4c9e3bd62ea5c09a3b11ebcb33e7
(cherry picked from commit 763393644a0a6e361f26dfe26f6dc0021579f011)