Handles transactions started immediately after completion

Unset mIsTransactionPending before invoking handleTxnResult
to allow service to start a new transaction.

Bug: 70387233
Test: Run a sample application that concurrently loads and unloads 2
nanoapps, verify both transactions succeed.

Change-Id: I465882dd8fd8fdd89b959642f6fdc2e452ce61a6