boringssl self tests: explicitly specify empty capabilities

If a service doesn't specify any capabilities in it's definition in the
.rc file, then it will inherit all the capabilities from the init.
Although whether a process can use capabilities is actually controlled
by selinux (so inheriting all the init capabilities is not actually a
security vulnerability), it's better for defense-in-depth and just
bookkeeping to explicitly specify that boringssl_self_test doesn't need
any capabilities

The list of capabilities was obtained via:
```
$ adb pull /sys/fs/selinux/policy /tmp/selinux.policy
$ sesearch --allow -s boringssl_self_test -c capability,capability2 /tmp/selinux.policy
```

Bug: 249796710
Test: device boots
Test: presubmit
Change-Id: I866222e2325e59d7e39d00db59df7b83efc657d9