Inroduce UNIFORM_OPS sampling.

Introduce strategy which uniformly samples OPs, without biasing in
favour of specific ops.
Use this strategy half of the time.

Test: atest RuntimeMessageCollectionTest && statsd_testdrive 10069
Bug: 143519689
Change-Id: I481d3a9ef9c66f04577b57432cecdf2c02055e58
diff --git a/atoms.proto b/atoms.proto
index 36a8b2c..bdd8964 100644
--- a/atoms.proto
+++ b/atoms.proto
@@ -9042,6 +9042,7 @@
         UNIFORM = 1;
         RARELY_USED = 2;
         BOOT_TIME_SAMPLING = 3;
+        UNIFORM_OPS = 4;
     }
 
     // sampling strategy used to collect this message