fs_mgr: split fstab parsing into separate file

Move fstab parsing into fs_mgr_fstab.c so that it compiles into a
separate compilation unit.  uncrypt links against it but doesn't
need to pull in the rest of the dependencies on fs_mgr.c.

Change-Id: I6bffe1cd42720151533e958028259d1931aaa4bf
diff --git a/fs_mgr/Android.mk b/fs_mgr/Android.mk
index 790598a..165ebd4 100644
--- a/fs_mgr/Android.mk
+++ b/fs_mgr/Android.mk
@@ -3,7 +3,7 @@
 LOCAL_PATH:= $(call my-dir)
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES:= fs_mgr.c fs_mgr_verity.c
+LOCAL_SRC_FILES:= fs_mgr.c fs_mgr_verity.c fs_mgr_fstab.c
 
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/include