android_filesystem modules can be included in APEX

android_filesystem modules can be included in APEX via the new
`filesystems` property. The filesystem images are placed at
./etc/fs/<modulename>.img.

Bug: 172413888
Test: m nothing

Change-Id: I215ca7a32ff1988a0de4e1f71397684e189839ea
diff --git a/apex/Android.bp b/apex/Android.bp
index 9e8c30d..e3a547c 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -7,6 +7,7 @@
         "soong-android",
         "soong-bpf",
         "soong-cc",
+        "soong-filesystem",
         "soong-java",
         "soong-python",
         "soong-rust",