1. Set APP_ABI:=all in almost all samples
2. Enhance hello-jni to report more ABIs
3. Fix warnings
Change-Id: I2851c42b77ef8225e32143d225edcf973633e782
The idea is to allow developing the NDK in the open, while
having the platforms and samples under a private branch until
the corresponding release are open-sourced.
Change-Id: Iee995fb6c4d3ee1387dea7486e599e079c9e4c6d