Reland "Fix dependency loop with flex"

I'm adding a prebuilt_build_tool module for flex so that it can be used
by genrules. But since flex uses flex during it's own build, and
prebuilt_build_tool can't handle the loop, explicitly change the name to
"flex_bin", and use stem so that the final binary is still called flex.

Bug: 128690776
Test: treehugger
Change-Id: I6a818cbd297f951552d2890374787eb40c074463
diff --git a/src/Android.bp b/src/Android.bp
index e3b3132..d916fb0 100644
--- a/src/Android.bp
+++ b/src/Android.bp
@@ -1,5 +1,9 @@
 cc_binary_host {
-    name: "flex",
+    // Not named "flex" because we need the flex module to build scan.l below,
+    // and we don't support dependency loops. The stem renames this back to
+    // 'flex' in the installed path
+    name: "flex_bin",
+    stem: "flex",
     stl: "none",
     srcs: [