Fix setAttachingSchedGroupLSP() to support use_fifo_ui

The method added in aosp/1249555 ("Start process of next activity with
top priority in advance") to set the priority of the newly-launched top
app's UI thread doesn't handle the use_fifo_ui=1 case.

By setting mSetSchedGroup it also prevents subsequent applyOomAdjLSP()
calls from fixing the priority, so on devices with the sys.use_fifo_ui
sysprop set, main thread may not actually use SCHED_FIFO. This is an
issue mainly for the initial launch of an app -- once it's moved to
another sched group and then back, the priority is adjusted correctly.

Test: set sys.use_fifo_ui, start a new app, and check thread priorities
      with `ps -lT <pid>`
Signed-off-by: Tomislav Novak <tnovak@meta.com>
Change-Id: Ic8afc2eb054717018d227263a93d9fcc25bfa180
(cherry picked from commit 68b35014e524dd20ac1537ad4cc55f004afe26a9)