Raise minimum API level to 16
Devices running API 15 account for less than 1/1000 of our active installs according to Google Play.
Removing the non-PIE 32-bit binaries for Tor and obfsproxy, which are only needed for API 15, will make room for the 64-bit PIE binaries. There may also be some conditional code we can remove. One of the test devices will have to be retired.