[Development] [Android] Raising minimum supported Android NDK version to r19

[Alexey Rochev]

Hello,
I'm currently investigating several problems with building Qt with
recent (r19 and r20) NDK versions. In NDK r19 Google added a new
toolchain, and we need to use it for some features to work (e.g.
LTCG). It would greatly simplify build system configuration if we
dropped support of previous NDK versions. Doing so will also require
us to drop support of android-g++ platform (GCC was removed in NDK
r18) and armv5 (removed in NDK r17). Also it will require clients to
use r19+ too. What do you think?

Alexey Rochev