Combine startPull and sendHeartbeat threads

This ensures that only one helper thread is created per subscription.
Previously, there could be up to two.

+ fixes thread sleep duration if the pulled atoms have different pull
frequencies
+ rename attemptWriteToSocketLocked to attemptWriteToPipeLocked

Test: atest statsd_test
Test: atest CtsStatsdHostTestCases:ShellSubscriberTest
Test: manual testing on Android Studio
Bug: 156678125
Change-Id: I7074bbba5981a591a30e8b70a1ad1d83eadfcc30