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/btf.h b/libc/kernel/uapi/linux/btf.h
index 475e2eb..21e7596 100644
--- a/libc/kernel/uapi/linux/btf.h
+++ b/libc/kernel/uapi/linux/btf.h
@@ -31,8 +31,8 @@
   __u32 str_off;
   __u32 str_len;
 };
-#define BTF_MAX_TYPE 0x0000ffff
-#define BTF_MAX_NAME_OFFSET 0x0000ffff
+#define BTF_MAX_TYPE 0x000fffff
+#define BTF_MAX_NAME_OFFSET 0x00ffffff
 #define BTF_MAX_VLEN 0xffff
 struct btf_type {
   __u32 name_off;