Require pip2 to be off for minimal size test case

PinnedStackTest#testEnterPipWithMinimalSize() has been
flaky after PiP2 flag was turned on, only failing sometimes
on first run, but always passing on retries.

Isolated ABTD forrest runs on the same CF target would pass,
and the test has been passing locally after numerous iterations too.
So until we figure out the root cause, we should require the flag be off
for this specific test case, so that new CLs don't have re-run the
Presubmits twice to have this test pass.

Other PinnedStackTests have been passsing, so we'll leave them on in
PiP2 as expected.

Bug: 380030822
Flag: com.android.wm.shell.enable_pip2
Test: atest PinnedStackTests#testEnterPipWithMinimalSize
Change-Id: Ia3198d60b903ccfbbc92bfef1585ab824d46e4f7