ANDROID: Fixes to locking around handle_lmk_event

get_task_struct used to reserve 'selected' outside rcu_read_lock block.
Remove the need for get_task_mm, removing the possibility of lock issues there.

Bug: 133479338
Signed-off-by: Jim Blackler <jimblackler@google.com>
Change-Id: I1399e2f669242c04e0e397bc09c987358aa97a0a