Add a no-op flag to mutable implicit PendingIntents

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 intents passed into
getPendingIntent() can be both explicit and implicit, so we added
a no-op FLAG_ALLOW_UNSAFE_IMPLICIT_INTENT to bypass the above block of
mutable implicit PendingIntents.

The flag is introduced in ag/21018466.

Bug: 236704164
Bug: 229362273
Test: TH passes
Change-Id: Ia26c8f92d1b4b50e04bc6b487619f54efc7d5a1d