Make mutable implicit PendingIntents immutable

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 PendingIntents here do not
get mutated, so we made them immutable.

Reviewers, please call out if this is not the case.

Bug: 236704164
Bug: 229362273
Test: atest MediaVolumePreferenceControllerTest
Change-Id: Ic5f701b504c0d8d0d0a44b002117ee5ef1c188f7