Add a mutability flag to the PendingIntent

It's soon going to be required for apps targeting S+ to specify
explicitly either FLAG_MUTABLE or FLAG_IMMUTABLE when creating
a PendingIntent. Thus, add a mutability flag to the
PendingIntent that doesn't specify it before.

Bug: 170602021
Bug: 170766590
Test: atest FrameworksNetTests
Change-Id: I56581033c0a1b450b6626112cdae5e5d9aac873f