Actually, IIRC, they didn't copy the signatures. When you build an android app, you use Oracle's JDK [and/or openJDK or some such] and get the signatures from that. Just like if you were using the JDK to create your own Java [non-Android] app. Otherwise, anybody writing Java code would be in the same boat.
Oracle has a copyright on the API. Oracle was trying to convert this to a "patent" on it. If Oracle had a patent [which they can't get], then Google would not have been able to create the underlying [Dalvik] from scratch re-implementation of the JVM.
If Oracle had won here, ironically, they would have had to open source the code to their database software. They port to Linux and it had GPL v2 [as does glibc, etc.]. Also, they use C, and the ISO C spec has a copyright.
If Oracle had won, anyone writing a C program would have had to make a royalty payment to ISO.
Further, the stdio.h that comes with glibc has a copyright. That doesn't prevent BSD from creating their own stdio.h [they are both built from scratch, even if they both define similar things to implement the POSIX specifications]. Actually, if Oracle had prevailed, all Linux implementations, gcc, glibc would be shut down because POSIX [specs] could never have existed. POSIX specs were a "clean room" reimplementation of the _specifications_ of interfaces and programs that had copyrights (i.e. AT&T had copyrights on the _Unix_ man pages for open/close/read/write and other system calls and utilities like ls/df/du, etc.)