Follow-up commit of aosp/2329897

This commit resolves comments addressed in aosp/2329897.
- Remove EX_UNSUPPORTED_OPERATION return from setDataWarningAndLimit()
- Update some comments
- Change some errors into assertions
- Correct the sequence of the arguments in EXPECT_EQ(), expected
  argument goes before the actual argument
- Print error or error message if API call fails

Bug: b/205762647
Test: atest VtsHalTetheroffloadTargetTest
Change-Id: Ied83dac5c33e06f6ad4ced18fbee785047b24c38