clobber wom: get correct most recent task when stem early short press

The expected double press behavior is to switch to the most recent app.

When "early short press" enabled for stem primary key, an ALL_APPS
launcher is launched upon first key-up envet. That means if we
query the most recent backgroud task at detected double press event,
we'll always get the ALL_APPS launcher, instead of the correct
"most recent app". So, we need to collect the "most recent app" info
earlier, before the ALL_APPS launcher of key-up and use that collected
info as the target task to swtich to at double press event.

Test: atest WmTests:StemKeyGestureTests
BUG: 297122524
Change-Id: I23f6e144b7265643980ba1d6619776c7058d9841