[pm] helper class to handle preferred activity

Also adds a helper class for resolving intent.

Some methods goes through mComputer so I have to keep them in PMS. We
will probably move more things into ComputerEngine later.

BUG: 200582135
BUG: 200616017
Test: manual
Change-Id: I1a0f0298bedd6d60e861a8e58b50c2a349e7e8fb