Add activitymanager debug option to freeze a process

This makes testing paths that rely on freezer such as compaction
etc doable in a simple way as well as allowing instrumentation
to triggering them.

Bug: 274177499
Test: am freeze [--sticky] <processname>
Test: am unfreeze [--sticky] <processname>
Change-Id: I832795b59d9a1078665a5474506732c3c7039abe