Add FLAG_IMMUTABLE for telephony pending intents.

Strict enforcement of the presence of FLAG_IMMUTABLE and FLAG_MUTABLE is
being enforced in S+.  Adding these flags to some pending intents in
Telephony to fix runtime crashes.

Test: build; manual regression test.
Fixes: 178716675
Fixes: 179239715
Change-Id: Ic631f3f716ec639c7a916e20e753c73845da391f