Get NDK python script tests running.

Imports weren't working in tests because the package had been created.
The Python "binaries" built by Soong don't seem to take their own
pkg_path into account, so I split the separate pieces of code here out
into their own packages.

Note that the ndk_api_coverage_parser tests do not actually pass
before or after this change (seems like it might be a
non-deterministic ordering issue in the attributes of the generated
output?), but they can at least be run now.

Test: pytest ndkstubgen
Test: pytest symbolfile
Test: pytest ndk_api_coverage_parser
Test: out/host/linux-x86/nativetest64/test_ndkstubgen/test_ndkstubgen
Test: out/host/linux-x86/nativetest64/test_symbolfile/test_symbolfile
Test: out/host/linux-x86/nativetest64/test_ndk_api_coverage_parser/test_ndk_api_coverage_parser
Bug: None
Change-Id: I2ac22f7ced7566e4808070f2f72fd04355846e0b