java_sdk_library: Create separate impl library

Most dependencies on java_sdk_library instances (JSL) simply reference
them by name and rely on the build to pick the correct part, impl,
stubs for a specific scope, headers or implementation jars or dex jars.
Generally, the build gets that decision right. If it does not the for
most of the components of the JSL there is a way to reference them
directly. However, that is not true for the implementation library as
that is created the JSL itself.

Access to the implementation library should be avoided as it opens up
the possibility that use will be made of an API that is not tracked and
maintained which could lead to compatibility issues. However, there is
a case where use of the implementation library is required and safe.
That is when it is being used from other libraries within the same APEX
as the JSL. In that case the JSL and its dependencies will be updated
together so compatibility is not an issue.

This change adds a new <module>.impl java_library that compiles the
implementation library in the same way as the JSL itself does. This will
allow the modules that are blocked from converting to JSL because of
this limitation to progress. Initially the library is only for use
for compiling but follow up changes will switch the rest of the build
from using the implementation library created by JSL to the separate
module.

It also adds the ability to give the separate .impl library a more
restrictive visibility than the JSL further limiting misuse.

Bug: 156618935
Test: m droid, flash, atest CtsProviderTestCases - sniff test
      tested that it fixes compilation issues with mainline module
Change-Id: Ifc4799feb4ada9dec8021efc13fe6a953bf2edd3