Refine NetworkMonitorTest#testReadAsString_StreamShorterThanLimit()

The characters of string may not only contain ASCII characters,
so the length of string may not the same as bytes.

Replace the string to byte[].

Bug: 154217644
Test: atest NetworkStackTests:NetworkMonitorTest
Change-Id: I3c1f826ef1be0302fdb19c294087464af8573105
Merged-In: I6552a6c8a291d54fc16bd3a1df3ab29a4481b20d
(cherry picked from commit 57096958)