Remove String.setCharAt().

The internal API String.setCharAt() breaks the assumption
that strings are really immutable. That in turn breaks
string compression invariants - compressible strings must
be compressed. This CL removes the String.setCharAt() API.

The method was used only in String.replace(char, char)
when we found a match, copying the string on first match.
Instead, introduce a new native method that does the whole
replacement with a single call when we find the first match.

StringReplaceBenchmark results on Nexus 6P, lower is better:
  timeReplaceCharNonExistent/EMPTY   41.93 ->   38.25 (-9%)
  timeReplaceCharNonExistent/L_16   114.90 ->   95.09 (-17%)
  timeReplaceCharNonExistent/L_64   419.97 ->  320.65 (-24%)
  timeReplaceCharNonExistent/L_256 1667.01 -> 1091.25 (-35%)
  timeReplaceCharNonExistent/L_512 3253.50 -> 2075.62 (-36%)
  timeReplaceCharRepeated/EMPTY      41.93 ->   39.58 (-6%)
  timeReplaceCharRepeated/L_16      114.87 ->   95.40 (-17%)
  timeReplaceCharRepeated/L_64     1267.29 ->  704.32 (-44%)
  timeReplaceCharRepeated/L_256    5139.14 -> 1361.80 (-74%)
  timeReplaceCharRepeated/L_512   10787.81 -> 2338.41 (-78%)
  timeReplaceSingleChar/EMPTY        41.78 ->   37.16 (-11%)
  timeReplaceSingleChar/L_16        449.54 ->  497.51 (+11%)
  timeReplaceSingleChar/L_64        942.08 ->  891.35 (-5%)
  timeReplaceSingleChar/L_256      2756.18 -> 2174.64 (-21%)
  timeReplaceSingleChar/L_512      5489.91 -> 3983.32 (-27%)

Test: testrunner.py --host
Test: run-libcore-tests.sh --mode=host
Test: testrunner.py --host with string compression enabled.
Test: run-libcore-tests.sh --mode=host with string compression enabled.
Bug: 31040547
Change-Id: I9cf0d5457182f0a33ca8251c29931d3eb624ae07