Make //apex_available:platform the default.

go/Android.bp mentions that ["//apex_available:platform"] is the
default, but currently it was []. This change does not create any
additional module variants.

(Noticed this for libz, I was expecting its apex_available to be
platform and not an empty list)

Test: TH
Change-Id: I9af06f813b1a1d7b716939874f469bd2e1ce4d14