Disallow missing api source files for java_api_library module

The purpose of this change is to enable filegroups to be taken as inputs
for `java_api_library` module `api_files` property. However, since
android.MaybeExistentPathForSource() does not support this, it needs to
be replaced with android.PathForModuleSrc(), which checks for the files'
existence.

Bug: 283006953
Test: go test ./java
Change-Id: I8a7d7f200f900219cc17243194a4c26071329ee6