Update language to comply with Android's inclusive language guidance

See https://source.android.com/setup/contribute/respectful-code for reference

Bug: 161896447
Test: refactoring CL, existing tests still pass. Verified by running
 `atest HkdfTest.java` in the corresponding test directory.
Change-Id: If6154e6d75eab4b2c97bdc08f5eebdba4cf45249