Add apex_available to control the availablity of a module to APEXes

apex_available property controls the availability of a module to APEXes.
For example, `apex_available: ["myapex", "otherapex"]` makes the module
available only to the two APEXes: myapex and otherapex, and nothing
else, even to the platform.

If the module is intended to be available to any APEX, then a pseudo
name "//apex_available:anyapex" can be used.

If the module is intended to be available to the platform, then another
pseudo name "//apex_available:platform" is used.

For now, if unspecified, this property defaults to ["//apex_available:platform",
"//apex_available:anyapex"], which means the module is available to everybody.
This will be reduced to ["//apex_available:platform"], when marking for
apex_available for existing modules are finished.

Bug: 139870423
Bug: 128708192
Test: m
Change-Id: Id4b233c3056c7858f984cbf9427cfac4118b2682