creating workflow for mirror::String compression

All-ASCII String characters are stored in 8-bit blocks
instead of 16-bit. The compression has not taken place, but all
workflow are in the code already (changing kUseStringCompression in
string.h file to TRUE will enable the feature)

Notes: Feature works on interpreter only without optimizing

Test art: m ART_TEST_INTERPRETER=true ART_TEST_OPTIMIZING=false
          test-art-host

Also tested with String tests from libcore/:
1. libcore.java.lang.StringTest
2. libcore.java.lang.StringBufferTest
3. libcore.java.lang.StringBuilderTest
4. libcore.java.lang.OldStringTest
5. libcore.java.lang.OldStringBufferTest

Memory improvement is 33% (from 6.03% to 4.03%, total String memory
from all apps per total memory of all apps) measured on Angler
with Hprof tools

Bug: 31040547
Change-Id: I9cc92c265ebf1305fc06b5fc33efd83797660cce
diff --git a/runtime/mirror/object_test.cc b/runtime/mirror/object_test.cc
index 0034220..b35a479 100644
--- a/runtime/mirror/object_test.cc
+++ b/runtime/mirror/object_test.cc
@@ -62,7 +62,7 @@
     Handle<String> string(
         hs.NewHandle(String::AllocFromModifiedUtf8(self, expected_utf16_length, utf8_in)));
     ASSERT_EQ(expected_utf16_length, string->GetLength());
-    ASSERT_TRUE(string->GetValue() != nullptr);
+    ASSERT_EQ(string->IsValueNull(), false);
     // strlen is necessary because the 1-character string "\x00\x00" is interpreted as ""
     ASSERT_TRUE(string->Equals(utf8_in) || (expected_utf16_length == 1 && strlen(utf8_in) == 0));
     ASSERT_TRUE(string->Equals(StringPiece(utf8_in)) ||