Fix String8::operator+

The LHS was ignored when using:
  String8 + String8
  String8 + (const char*)

Add unit tests for above.

Bug: 2898473
Change-Id: Ic8fe7be668b665c36aaaa3fc3c3ffdfff0fbba25
diff --git a/libs/utils/tests/Android.mk b/libs/utils/tests/Android.mk
index b9f206a..725de9c 100644
--- a/libs/utils/tests/Android.mk
+++ b/libs/utils/tests/Android.mk
@@ -7,7 +7,8 @@
 # Build the unit tests.
 test_src_files := \
 	ObbFile_test.cpp \
-	PollLoop_test.cpp
+	PollLoop_test.cpp \
+	String8_test.cpp
 
 shared_libraries := \
 	libz \
@@ -41,4 +42,4 @@
     $(eval include $(BUILD_EXECUTABLE)) \
 )
 
-endif
\ No newline at end of file
+endif