Update to android-mainline kernel headers v5.4.

Update generate_uapi_headers.sh to checkout the android mainline
kernel. Also, add a small modification to look for the kernel directory
in common not linux-stable.

Remove deprecated android headers from android/uapi/linux. Also,
remove f_accessory.h since it's in the android mainline kernel.

Test: Builds and runs on walleye.
Change-Id: Ia371305e19f56e6bcc2db6d5b4d299819f07ffc6
diff --git a/libc/kernel/uapi/linux/io_uring.h b/libc/kernel/uapi/linux/io_uring.h
index c5055ba..7ae31df 100644
--- a/libc/kernel/uapi/linux/io_uring.h
+++ b/libc/kernel/uapi/linux/io_uring.h
@@ -34,6 +34,7 @@
     __u16 poll_events;
     __u32 sync_range_flags;
     __u32 msg_flags;
+    __u32 timeout_flags;
   };
   __u64 user_data;
   union {
@@ -58,6 +59,7 @@
 #define IORING_OP_SYNC_FILE_RANGE 8
 #define IORING_OP_SENDMSG 9
 #define IORING_OP_RECVMSG 10
+#define IORING_OP_TIMEOUT 11
 #define IORING_FSYNC_DATASYNC (1U << 0)
 struct io_uring_cqe {
   __u64 user_data;
@@ -96,10 +98,12 @@
   __u32 flags;
   __u32 sq_thread_cpu;
   __u32 sq_thread_idle;
-  __u32 resv[5];
+  __u32 features;
+  __u32 resv[4];
   struct io_sqring_offsets sq_off;
   struct io_cqring_offsets cq_off;
 };
+#define IORING_FEAT_SINGLE_MMAP (1U << 0)
 #define IORING_REGISTER_BUFFERS 0
 #define IORING_UNREGISTER_BUFFERS 1
 #define IORING_REGISTER_FILES 2