apex: make allowed_files prop overridable

Because override_apex can modify the contents of the base apex,
allowed_files (which describes the contents) should be overridable.

Bug: 159503079
Bug: 159392784
Bug: 158169437
Test: m (soong test added)
Change-Id: I12744b0465dc3cfc90a66643867e65b4092cd0f7