Offer to wait until broadcasts have drained.

We've seen evidence of lab devices racing with other apps that are
using cache space immediately after tests wipe it clean, which can
cause test failures.  To mitigate this, try our best to wait for the
device to go "idle" by watching for broadcast queues to fully drain.

Also improve javadocs along the way.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.StorageHostTest
Bug: 37486230, 37566983, 37913442, 37914374
Change-Id: I4d430db443b6fa6d33a625fe07b90279b5d51c12