Improve TestLooperManager check on execute()

Looper.isCurrentThread() does exactly what we need.

Checking the same condition via a side effect on
Looper.myLooper() is harder to read.
It's also less reliable, because I caught some (terrible)
tests manipulating ThreadLocals.

Bug: 379472827
Change-Id: Id67eddcb57329cb42b5b4ba6b09595ec720f6cdb
Flag: android.os.message_queue_testability
Test: android.os.cts.TestLooperManagerTest