init: Allow SETPCAP for dropping bounding set.

This is required for https://android-review.googlesource.com/#/c/295748
so that init can drop the capability bounding set for services.

Bug: 32438163
Test: With 295748 and a test service using ambient capabilities.
Change-Id: I57788517cfe2ef0e7a2f1dfab94d0cb967ede065