Fix issues caught by Android Lint am: d0d5415ff4 am: fa32132f9a am: 18e63309dd am: 162b53f9d2

Original change: https://android-review.googlesource.com/c/platform/external/androidplot/+/1346923

Change-Id: I520f4cf7aea39f713fa3be955d472b143d488a2f