Add ActivationType to EventActivation

This allows setting one metric with both IMMEDIATE and ACTIVATE_ON_BOOT
EventActivations

Also, if an on-boot Activation that is already active gets another
activation signal, ignore it.

Bug: 128880263
Fixes: 128880263
Test: statsd_test
Change-Id: I8d483882836c9abc184230b4a70d4734d49d93c3