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/wait.h b/libc/kernel/uapi/linux/wait.h
index d14a597..385e882 100644
--- a/libc/kernel/uapi/linux/wait.h
+++ b/libc/kernel/uapi/linux/wait.h
@@ -30,4 +30,5 @@
 #define P_ALL 0
 #define P_PID 1
 #define P_PGID 2
+#define P_PIDFD 3
 #endif