Avoid app pinning requests if the Task is already locked

Bug: 378514614
Test: LockTaskControllerTest
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b4945cf98c97121821c178713a613a6a6e830c3a)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a3240bf2ba1587b808b4314dd2e32c624b57c8da)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:326c70b0fd73c3769a252b83e1b3c18069f28b87)
Merged-In: Ic5d2125d39e5e98301d79c8f5aa8986a44e06794
Change-Id: Ic5d2125d39e5e98301d79c8f5aa8986a44e06794