ANDROID: binder: pass desired priority by reference

Avoid making unnecessary stack copies of struct binder_priority and pass
the argument by reference instead. Rename 'desired_prio' to 'desired' to
match the usage in other priority functions.

There is no functional impact from this patch.

Bug: 148101660
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I66ff5305296e7b9dba56ed265236f2af518f66e0