DropBoxTests: ensure DropBoxManager.addText is called at fixed interval

The test meant to send DropBox entries at a certain interval in order to
verify the throttling behavior, but in fact it's calling addText in a
loop with a constant delay before each call, making the timing
unpredictable.

This CL takes the time spent in addText into consideration, to make sure
it's being called at the given interval.

Bug: 177029550
Test: CtsDropBoxManagerTestCases
Change-Id: If1040113da20839b4b68ea5a0d752b52e19e15dd
1 file changed