Add common string utilities to libbase.

These are useful outside of ART. Nothing changed (aside from fixing
Trim to not segfault on empty strings), so ART should be able to move
to using these.

Change-Id: Id026ebffe8d31f784a91834786ab189680b13a0f
diff --git a/base/Android.mk b/base/Android.mk
index 3b64ab0..0e1a9b6 100644
--- a/base/Android.mk
+++ b/base/Android.mk
@@ -19,10 +19,12 @@
 libbase_src_files := \
     file.cpp \
     stringprintf.cpp \
+    strings.cpp \
 
 libbase_test_src_files := \
     file_test.cpp \
     stringprintf_test.cpp \
+    strings_test.cpp \
 
 libbase_cppflags := \
     -Wall \