Previously, Animal Sniffer checking of spring-boot-loader-tools
was disabled as it failed with an NPE. This has been fixed in
Animal Sniffer 1.15.

This commit upgrades Animal Sniffer to 1.15 and adds the necessary
annotations to suppress failures for safe usage of sun.* and Java 7
APIs.

Note that UsesUnsafeJava has been copied from spring-boot and made
package-private. This retains the clearer intent of the custom
annotation (versus @IgnoreJRERequirement) while avoiding the change
in the build order that would be necessary for
spring-boot-loader-tools to use the annotation from spring-boot.

Closes gh-5284