Add basic exFAT support.

Several partners have been requesting exFAT support.  Android doesn't
natively support exFAT, but we're at least willing to try mounting an
exFAT filesystem if we detect the Linux kernel supports it, and if
helper binaries are present.

This CL is simple scaffolding, and it provides no actual
implementation of exFAT.

Test: builds, boots
Bug: 67822822
Change-Id: Id4f8ec3967b32de6e1c0e3c4b47fe6e43a6291ab
diff --git a/Android.bp b/Android.bp
index aca6493..0a45395 100644
--- a/Android.bp
+++ b/Android.bp
@@ -115,6 +115,7 @@
         "VolumeManager.cpp",
         "authorization_set.cpp",
         "cryptfs.cpp",
+        "fs/Exfat.cpp",
         "fs/Ext4.cpp",
         "fs/F2fs.cpp",
         "fs/Vfat.cpp",