Perfprofd: Add general event config to Config

Add general event configuration that allows more than instruction-count
sampling.

Add a lightweight syntax for event sets to the ConfigReader. Event
sets can be specified as

 * -e_{eventname}(,{eventname})*@{sampling_period}={ignored}
 * -g_{eventname}(,{eventname})*@{sampling_period}={ignored}

Also change the cmdline version to use a fresh ConfigReader every
time.

Add a test of the ConfigReader.

Bug: 110555909
Test: mmma system/extras/perfprofd
Test: perfprofd_test
Change-Id: I3d57d0d4fb2ce9fa7127f645d1a277033c88f1c4
4 files changed