Read all response to generate enough traffic while testing

Currently, testUidTagStateDetails relies on the test URL return
enough data (~100k) to ensure untagged traffic is not too much
to cause flaky. But aosp/2575590 change it to read only first few
bytes to speed up the test which would break this assumption.

Thus in this change:
1. Revert partical changes of aosp/2575590
2. Use BufferedInputStream to speed up test instead, which speed up
   testUidTagStateDetails from 2032ms to 902ms in local trials.

Test: atest CtsNetTestCases:NetworkStatsManagerTest
Bug: 289112440
Change-Id: I1b32d9e069a7b10664065e138c75c6bfe7928f23