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/if_bridge.h b/libc/kernel/uapi/linux/if_bridge.h
index 31683c1..03cc1c4 100644
--- a/libc/kernel/uapi/linux/if_bridge.h
+++ b/libc/kernel/uapi/linux/if_bridge.h
@@ -190,6 +190,7 @@
 #define MDB_PERMANENT 1
   __u8 state;
 #define MDB_FLAGS_OFFLOAD (1 << 0)
+#define MDB_FLAGS_FAST_LEAVE (1 << 1)
   __u8 flags;
   __u16 vid;
   struct {