Add back the api_srcs argument

This arg provides the ability to pass sources to the metalava
invocations that are part of a java_sdk_library without affecting
the runtime library.

This is useful in various scenarios, e.g.: when javadoc in some source
files affects the metalava behavior. For example, the javadoc-generation
javadoc tags @paramDoc, @returnDoc etc. Passing these as a library does
not work because class files do not contain javadoc.

Bug: 186197911
Test: manually inspect module stub src for doc enhancements
Merged-In: I2ce98f891283514e1896c52d9335550c1f5435fb
Change-Id: I2ce98f891283514e1896c52d9335550c1f5435fb