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
Merged-In: I8c2ca45c8f066cabf3247b9652a7094cefc047a9
(cherry picked from commit 7eff3a8f)