capabilities: Check ambient caps, last valid runtime cap.

Partners have expressed interest in using the 'capabilities' keyword
in init, so make the code more resilient:
-Check that ambient capabilities are supported by the kernel.
-Check that the last valid cap at runtime is not higher than what's in
kernel headers.
-Check that the user is not requesting a capability present in kernel
headers but not supported by the kernel at runtime.
-Don't attempt to drop bounding set capabilities not supported at
runtime.

This CL also fixes a small bug where < should have been used instead of
<=, and uses 'static' instead of anonymous namespaces.

Bug: 32438163
Test: Use a test service that uses capabilities.
Test: Apply in internal tree and test with angler and rild.

Change-Id: Ia271cc7eb389d1d526d61f897261e4bac4d19e5d