Allow tests to change UiAutomation flags.

Changing Instrumentation to modify the existing
UiAutomation when one with different flags is requested.

Also checking if UiAutomation is destroyed before disconnecting at the
end of a test.

Bug: 27365874
Bug: 27864279
Change-Id: Ia6efc4f89cc30dbe29286f30f8e9cb8f6fdb4712