Revert "Revert "Set FLAG_IMMUTABLE flag on FirstScreenBroadcast PendingIntent. <snip>

This reverts commit 1aa95b84.

Reason for revert: automerger got confused by the revert of a merge, and
skipped it downstream.

NB: resending this as a standalone patch, NOT generated via Gerrit's
"revert" functionality, to avoid confusing AM further (I'm told there
are hidden tags involved).

IGNORE_MERGE_CONFLICT_CHECK=="trying to fix the build, see the rest of the topic"

Since the revert was skipped downstream, using a merged-in tag to also
stop this change from propagating.

Bug: 183927137
Merged-In: I8d8e38a1aa6bdf13879d460cfa84cabe6c6bb1f2
Change-Id: I541eeb1cc1b589dc0134a5e2e4c16be9f658fbbb