Add installable property to apex module type

When set to false, the apex is built but not installed. Useful for not
polluting the system image with test APEXes when doing mma.

Bug: 120960303
Test: add 'installable: false' to apex.test under system/apex/tests/Android.bp
m; check that out/..../system/apex/apex.test.apex does not exist.

Change-Id: I383d5cdcc8aec705b6a5778dbd07233471d289d0