Fix typo in AAPT2 error message

AAPT2 - and 1 - print "String pool is unitialized."
I found this out when I saw the error and tried to look for it in Code Search only to get no results because it's spelled wrong.

Change-Id: I66e2dcada0b1441d9f21aebded1ef95af09e7ac1
Flag: EXEMPT log only update