Make soong create sourcepath dirs

Sourcepath is set to dir of the Android.bp, but that dir does not
necessarily have any inputs for the metalava invocation defined in that
Android.bp. This leads to problems when metalava tries to read a
non-existent dir, so create it first.

Bug: 153703940
Test: m sdk dist
Change-Id: I8c2ca45c8f066cabf3247b9652a7094cefc047a9