Convert appsearch to java_sdk_library

AppSearch build file only defined a subset of the stubs and api
tracking, which prevents making progress on creating a correct "merged"
current.txt. Convert it to the standard build setup modules should use.

Implementation wise, this is a little bit special in that it's somewhere
between a module and not. The implementation library requires platform
internals to compile. The stubs do not, and we rely on this to make
the build work at all -- via impl_only_libs we compile the stubs
*without* framework-minus-apex, which allows framework-minus-apex to
depend on the stubs.

Bug: 169304493
Test: m checkapi
Change-Id: I10a331bb6df56072cb9e9ada99ae47c5446869fd