fix: Use String.trim instead of String.strip

[aababilov]

String.strip is available only from Java 11 while String.trim is more
portable.

The difference is tiny and we can neglect it in our context:

• trim() removes all characters that are less than or equal to 'U+0020'
  (the space character)
• strip() uses Character.isWhitespace function, so it removes fewer
  characters

[aababilov]

String.strip is philosophically better but we don't have it at Google (ugh, that old Java).

@barbeau Do you find acceptable if we make our upstream compatible with older Java 8?

[barbeau]

Do you find acceptable if we make our upstream compatible with older Java 8?

That's fine by me to revert back to String.trim(), although to guarantee Java 8 compatibility I believe we should then change the sourceCompatibility in build.gradles from 11 to 8, and change the JDK that the CI is running on from 11 to 8 as well (see #593).

[barbeau]

👍

[aababilov]

#759 to downgrade the rest of the code.