Added support for pulled atoms for stats-log-api-gen

Currently, stats-log-api-gen allows statsd clients to send events for
pushed atoms to statsd using a simple auto-generated function call.
Allow support for pulled atoms so that clients no longer have to
hand-construct StatsEvent objects by calling StatsEvent write methods
directly and also making sure that the appropriate annotations are
added. This support is for Java clients only for now.

Bug: 160368804
Test: Ran `m statslog-framework-java-gen` and completed successfully
Test: Ran `m` and completed successfully
Test: Ran `m stats-log-api-gen-test && out/host/linux-x86/nativetest/stats-log-api-gen-test/stats-log-api-gen-test`
Test: Ran `atest CtsStatsdHostTestCases:android.cts.statsd.atom` and all passed except for CTS tests related to a SIM card because the testing device does not have a SIM card
Change-Id: Ie2cbb298690ce71ff7fe7457b17b9d7bdbb24f6b