Implement issue #30977956: Enable Instrumentation tests for multi-process apps

New android:targetProcess attribute on <instrumentation> allows you to
specify the processes the instrumentation will run in.

This reworks how instrumentation is run in the activity manager to better
formalize its state and semantics, allowing us to keep track of it across
multiple processes.  This also clearly defines what happens when multiple
instrumentations are running at the same time, which is useful for writing
CTS tests that test the instrumentation APIs themselves.

Adds a couple new APIs to Instrumentation that helps with the new
situation where instrumentation can run concurrently in multiple processes.

Test: new CTS tests added (textXxxProcessInstrumentation in
ActivityManagerTest.java in cts/tests/app/src)

Change-Id: I2811e6c75bc98d4856045b2f0a45fb24af5d366f