Intercept JNI invocation of String.<init> methods.

Intercept JNI invocation of String.<init> methods.

libmono uses JNI AllocObject and CallNonvirtualVoidMethod to create and
initialize a string instead of using the recommended NewObject. This
change adds an intercept to change the String.<init> call to a
StringFactory call instead. Then, it uses the object id of the original
string object referrer and maps it to the result of the StringFactory.

Bug: 21288130

(cherry picked from commit 15e9ad1d028d7f12cb598b075453173532a00d91)

Change-Id: I3421c43722c07397da4a398c2ca9110e1d40bcfa