Revert "Improve libcore_util_CharsetUtils performance."

Revert submission 1449216

Reason for revert: Bug: 171537201
Reverted Changes:
I03d2420b2:Improve libcore_util_CharsetUtils performance.
I3dc453ce9:Improve mirror::String implementation.

Change-Id: I1df18b4421f3b37b84a5e54c908662538fc13f79
1 file changed