Fix mainline APK build

The mainline APK build builds a variant of PermissionController,
but if it is apex_available only to the apex
it can't be installed outside the apex.  Add //apex_available:platform
to restore the implicit platform availability that was present
before I44a3ef4030bd51c4ffbe84554a1f7bfcc4430bef

Bug: 147364041
Bug: 158059172
Bug: 232805504
Test: mainline build
Change-Id: Id389c93f53d1a1fd75eff636339f827fc8c64ff5
1 file changed