bazel apex: clean up gen_ndk_usedby_apex.sh to work in bazel sandbox.

This script is responsible for generating <module>_using.txt, which is a
file containing all NDK symbols used by the shared libraries in an APEX.

This script doesn't work as-is in the bazel sandbox; namely it tries to
create tmp directories in read only input dirs, for example:

"mkdir: cannot create directory 'bazel-out/android_target-fastbuild-ST-7fe6d3cd3905/bin/packages/modules/adb/apex/com.android.adbd_staging_dir/tmpUnzipped': Permission denied"

The staging dir inputs are also symlinks, so the `find` call needs a new
-L flag to follow symlinks.

This CL fixes that by using a tmpdir, and also refactors the script to
be less fragile.

I'm not a fan of the camelCase, but I'll save that for another change.

Test: presubmits
Bug: 239081455
Bug: 257226023
Fixes: 257226023
Change-Id: I1d0fe93c47c92d046e72c0ab32014a5dc8bbd597