java_sdk_library: Allow api dir to be specified

For modules that provide API surfaces in addition to the standard
current, test and system it is useful to be able to specify the
directory containing the api's .txt files to make it easy to create
multiple API surfaces from within the same Android.bp file. e.g. This
is useful for conscrypt, icu and libcore to manage their intra core
and core platform APIs.

Bug: 145998881
Test: m checkbuild
Change-Id: I753631d9b6993fbf30019fef5c052a9429e519de