[native] Clean up detect_abis.sh

Summary:
Relevant Linear issue here. D4717 was my original code to fix this, but it ended up breaking yarn react-native run-android.

Turns out the problematic code was writing if [[ ! $found ]] instead of if [[ "$found" = false ]] since in bash ! returns true for any nonempty string (and found is nonempty even when it's equal to false). Once I changed that, yarn react-native run-android builds successfully.

Test Plan: Along with ShellCheck, ran yarn react-native run-android successfully.

Reviewers: atul, jon, karol

Reviewed By: jon

Subscribers: ashoat, tomek, adrian

Differential Revision: https://phab.comm.dev/D4851