Make PendingIntent immutable and correct Settings package name

ConnectivityService puts up some notifications with pending
intents, but these pending intents are mutable that content can
be changed by someone. So make these pending intents to be
immutable.

Some OEMs have their own Settings package. Thus, need to get the
current using Settings package name instead of just use default
name "com.android.settings".

Bug: 154928507
Test: atest FrameworksNetTests
Change-Id: I02e3277358623400aa03dc8996af3d7c46a8ce76
Merged-In: I02e3277358623400aa03dc8996af3d7c46a8ce76