processor: extract a public ReadTrace() function from shell

Clients are asking for this :)

Bug: 142310140
Change-Id: Id035f857d779b4a5d2b854079d2fdb3f650073a9
diff --git a/Android.bp b/Android.bp
index dc329a6..58fc8ca 100644
--- a/Android.bp
+++ b/Android.bp
@@ -4017,6 +4017,7 @@
     "src/trace_processor/proto_trace_parser.cc",
     "src/trace_processor/proto_trace_tokenizer.cc",
     "src/trace_processor/raw_table.cc",
+    "src/trace_processor/read_trace.cc",
     "src/trace_processor/row_iterators.cc",
     "src/trace_processor/sched_slice_table.cc",
     "src/trace_processor/slice_table.cc",