Reduce binary size

[barsoosayque]

As per the current version (0.2.2), the binary size (all ABIs combined) is about 12MB, which is huge. It is 100% possible to reduce that. Three things worth mentioning:

• There is a binary size optimization flag in the configure script of FFmpeg. It disables performance optimizations, so it can be potentially harmful.
• Build libgdx-oboe.so with LTO optimizations. I believe, this requires FFmpeg libs to be statically built, but that's not a problem.
• Incorporate strip as a last built step; however, if I'm not mistaken, the NDK toolchain already does that.

[barsoosayque]

Here are some results:

• I wasn't able to link static ffmpeg libraries, so LTO wasn't really an option here.
• Binary size optimization flag didn't do much.
• Using -O2 -flto=full -ffunction-sections -fdata-sections -fvisibility=hidden cxx/c flags along with --as-needed link flags were useful.
• strip was super useful.

All in all, I got the library size cut down to 6.7 MB (-45%). I did everything I knew to reduce size, so if there any other method, reopen this issue / PR it !