Add installable property to apex_key

Setting the property to false prevents the key from being installed.
Useful for testing keys.

Bug: 122042717
Test: add 'installable: false' to the apex_key
'com.android.apex.test_package.key'. mma under
/system/apex/apexd/apexd_testdata. The key is not found under
out/target/product/..../system/etc/security/apex

Change-Id: Ibf70e4e8ea5e73432d06b1c4050df531eaafc85e