Copy StringPool to framework

As a part of libcore internal api cleanup, StringPool class is being
removed from SystemApi surface. The only user of StringPool is
JsonReader from framework, so copy StringPool there.

Bug: 154796679
Test: m update-api
Change-Id: I466038ac55fd11b348dc83355edb0ea80ca3f6b5
diff --git a/non_openjdk_java_files.bp b/non_openjdk_java_files.bp
index 4e37283..59a78e3 100755
--- a/non_openjdk_java_files.bp
+++ b/non_openjdk_java_files.bp
@@ -224,7 +224,6 @@
         "luni/src/main/java/libcore/icu/ICU.java",
         "luni/src/main/java/libcore/icu/LocaleData.java",
         "luni/src/main/java/libcore/icu/TimeZoneNames.java",
-        "luni/src/main/java/libcore/internal/StringPool.java",
         "luni/src/main/java/libcore/io/AsynchronousCloseMonitor.java",
         "luni/src/main/java/libcore/io/ForwardingOs.java",
         "luni/src/main/java/libcore/io/IoBridge.java",
@@ -353,6 +352,7 @@
         "luni/src/main/java/libcore/icu/CollationKeyICU.java",
         "luni/src/main/java/libcore/icu/DateUtilsBridge.java",
         "luni/src/main/java/libcore/internal/Java9LanguageFeatures.java",
+        "luni/src/main/java/libcore/internal/StringPool.java",
         "luni/src/main/java/libcore/io/ClassPathURLStreamHandler.java",
         "luni/src/main/java/libcore/io/BlockGuardOs.java",
         "luni/src/main/java/libcore/io/BufferIterator.java",