Remove floating-point time functions from StopWatch

Converting time units between floating-point milliseconds to integral
micros and floating-point seconds is error prone and leads to subtle
rounding bugs. Thus, consolidate all callers on microseconds and delete
the older methods.

Test: cd system/netd && atest .
Change-Id: Ief6ad4dbd21f442bfd65e78d49548677f3172a53