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/serio.h b/libc/kernel/uapi/linux/serio.h
index 72149ee..5ef9de4 100644
--- a/libc/kernel/uapi/linux/serio.h
+++ b/libc/kernel/uapi/linux/serio.h
@@ -76,4 +76,5 @@
 #define SERIO_EGALAX 0x3f
 #define SERIO_PULSE8_CEC 0x40
 #define SERIO_RAINSHADOW_CEC 0x41
+#define SERIO_FSIA6B 0x42
 #endif