Make android:cantSaveState an optional feature.

This doesn't make sense on things like watches and appliances,
so make this an optional feature that the device must enable.
If the feature is not set, then the system will ignore
the app's request.

Bug: 76213401
Test: atest CtsAppTestCases:ActivityManagerProcessStateTest
Change-Id: I91abf9d86ec14fa632e3bcc83c4a3febade5d2e4