Simpleperf: separate ThreadTree from SampleTree.

It is a preparation for unwinding in record command.

Bug: 22229391
Change-Id: I2f5c6df3f7332d4c98b61c7f9f922456b5bbeaaa
diff --git a/simpleperf/Android.mk b/simpleperf/Android.mk
index 278b96e..35bec54 100644
--- a/simpleperf/Android.mk
+++ b/simpleperf/Android.mk
@@ -46,6 +46,7 @@
   record.cpp \
   record_file_reader.cpp \
   sample_tree.cpp \
+  thread_tree.cpp \
   utils.cpp \
 
 libsimpleperf_src_files := \