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 (cherry picked from commit 31f5dd40886a696c7925feb53c4aa7cc9adf07c3)