[Multi-user] Fix BMS checkstyle

Now that checkstyle runs a preupload hook for frameworks/services, when
we move BMS -> UserBMS, checkstyle will complain of errors because it
sees it as a new file.

This CL just fixes checkstyle complaints (mostly adding javadocs,
variable naming, operand placement, etc.) for BMS so the CL that creates
UserBMS can have as little changes as possible to ease review.

There is no change in functionality in this CL.

Bug: 118520567
Test: atest RunFrameworksServicesRoboTests
Change-Id: I1118d6deef83d1abfbc8fd94883d7d162a4399cb