Bug 35349893
While executing postponed transactions, it was possible for
Fragments to call commitNow() or executePendingTransactions()
and therefore causing disallowed reentrancy.
This CL now prevents reentrancy while executing postponed
transactions.
Test: I11e10f4d9868a31b8b1616a051e381fe8c7af331
Change-Id: Icb0d4386657c35b4edf34f95dc48aa694e66b298