Add a no-op flag to a mutable implicit PendingIntent

Starting from target SDK U, we will block creation of mutable
PendingIntents with implicit Intents because attackers can mutate the
Intent object within and launch altered behavior on behalf of victim
apps. For more details on the vulnerability, see go/pendingintent-rca.

From a quick analysis, we concluded that the intent inside the
PendingIntent is only mutated inside startIntentSenderWithRestore()
which makes it possible to delay its creation and make it immutable.
However, this introduced test failures in SimpleSaveUiActivityTest.

To preserve the current behaviour, we decided to add a no-op flag
FLAG_ALLOW_UNSAFE_IMPLICIT_INTENT to the mutable implicit PendingIntent.
This flag is introduced in ag/21018466.

Bug: 236704164
Bug: 229362273
Test: atest CtsAutoFillServiceTestCases
Change-Id: Ia4e883de3c1df23ae645c8477b7010bd8086e00b