init: rename init_first_stage

In preparation for splitting first stage init from the rest of the
init executable, rename these files which are specifically involved in
first stage mount operations to a more appropriate name.

Test: builds
Change-Id: I8a2d4e8c7e1deea1bab45cc8e738727bc2ceb3e5
diff --git a/init/Android.bp b/init/Android.bp
index 5bbb7a0..d3305b9 100644
--- a/init/Android.bp
+++ b/init/Android.bp
@@ -104,9 +104,9 @@
         "devices.cpp",
         "epoll.cpp",
         "firmware_handler.cpp",
+        "first_stage_mount.cpp",
         "import_parser.cpp",
         "init.cpp",
-        "init_first_stage.cpp",
         "keychords.cpp",
         "log.cpp",
         "parser.cpp",