Fix potential block in InputReader

The 'dispatchExternalStylusState' is a callback function for
'ExternalStylusInputMapper' when the stylus device state is changed,
it's unnecessary to hold the lock again because it already hold when
processing the raw event.

Bug: 187253236
Test: atest inputflinger_tests
Change-Id: I19a2ef5f4c7a5afb622691ef7ec532f8d4dbb7ed