Use Locale.US where appropriate

Applicable for strings not intended for user consumption.

A %d string format code can generate eastern-arabic numerals
for users with an arabic locale.

V/k9      (20763): conn1103774136>>> 5 UID SEARCH ٦٤٦:٦٧٠ NOT DELETED
V/k9      (20763): conn1103774136<<<#5# [BAD, Invalid Search criteria]
E/k9      (20763): synchronizeMailbox
E/k9      (20763): com.fsck.k9.mail.store.ImapStore$ImapException: Command: UID SEARCH ٦٤٦:٦٧٠ NOT DELETED; response: #5# [BAD, Invalid Search criteria]