init: Drop inheritable capabilities when switching uids

This change explicitly drops all inheritable capabilities (and, by
extension, ambient capabilities) when there are no explicit capabilities
being set by a service and the user is changed. This prevents Android
running in a container from accidentally granting extra capabilities to
services.

Bug: 69320306
Test: aosp_sailfish still boots
Test: sailfish:/ $ grep Cap /proc/`pidof android.hardware.audio@2.0-service`/status
      CapInh: 0000000000000000
      CapPrm: 0000000000000000
      CapEff: 0000000000000000
      CapBnd: 0000003fffffffff
      CapAmb: 0000000000000000
Test: sailfish:/ $ grep Cap /proc/`pidof logd`/status
      CapInh: 0000000000000000
      CapPrm: 0000000440000000
      CapEff: 0000000440000000
      CapBnd: 0000003fffffffff
      CapAmb: 0000000000000000
Test: Android in Chrome OS still boots
Test: localhost ~ # grep Cap /proc/`pidof android.hardware.audio@2.0-service`/status
      CapInh: 0000000000000000
      CapPrm: 0000000000000000
      CapEff: 0000000000000000
      CapBnd: 000000006daefdff
      CapAmb: 0000000000000000
Test: localhost ~ # grep Cap /proc/`pidof logd`/status
      CapInh: 0000000000000000
      CapPrm: 0000000040000000
      CapEff: 0000000040000000
      CapBnd: 000000006daefdff
      CapAmb: 0000000000000000

Change-Id: I9218f2e27ff4fb4d91d50f9a98c0fdb4e272952c