DPMS: Enforce max byte length not character length

The Binary XML serializer doesn't support strings with a *byte*
length > 65k, but the DPMS code actually checked for *character*
length.

Test: android.devicepolicy.cts.AffiliationIdsTest#setAffiliationIds_idTooLong_throws
Bug: b/336550683
Flag: EXEMPT Bugfix

Change-Id: Ife885da75206d8000e3c0f247386685dc8d0ea76