Revert "Improve mirror::String implementation."

Revert submission 1449216

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

Change-Id: I5b1df73894c040017a7a64708c9904431c6cfe93
1 file changed