Previously, if generate_hiddenapi_lists.py was invoked without any
custom flag files, e.g. hiddenapi-unsupported.txt, then it would fail.
This change allows it to be used for generating the hiddenapi lists
for modules that do not have any custom flag files,
e.g. framework-sdkextensions
Bug: 179354495
Test: verified that the monolithic out/soong/hiddenapi/... files are
unchanged by this change
Change-Id: I455d453024c9f06ed59cbc1e9838234f8b7c7317
Also, creates a python_binary_host module for generate_hiddenapi_lists
and uses that when constructing the build rule rather than using the
file directly.
Bug: 177317659
Test: m droid
Verified that hiddenapi files (both aggregated ones and for the
individual modules) are not affected by this change.
Change-Id: Ia11bb203ce5a74740d35f1b7e86716e15aad336e
