Support multiple instrumentation clients

Changes Instrumentation::ConfigureStubs to support multiple clients
that need different levels of instrumenation. A client is identified
by a string key used to save the desired instrumentation level.

Also adds regression gtest instrumentation_test and some cleanup.

Bug: 19829329
Change-Id: I1fc24a86fcb7cb46d4be806895376c25cc0a0b3c