UserManagerService: Ignore prepare user storage errors for emulator builds

Ignore failures to prepareUserStorageInternal for emulator builds because
the emulator image's user records seems to report that they are a new
user despite having an on-disk user. The very feature was implemented to
prevent early errant per-user directory creation conditions during OEM OTA
updates which is inapplicable for the current development testing suite.

Test: build and boot to setup
Signed-off-by: Daniel Jacob Chittoor <djchittoor@murena.io>