[Android] Stop checking if RegisterNativesImpl() returns >= 0.

RegisterNativesImpl() returns a bool, so the comparison was always going
to return true.

Do the reasonable thing and just rely on the function's return value
without comparing it to anything.

(cherry picked from commit 592339ff4b176c524ba916b218bf025a51c03958)
Merge pull request #2586 from rakuco/v9-registernatives-return-value

[Android] Stop checking if RegisterNativesImpl() returns >= 0.