Enable Android build in CI, update NDK to 17c.

This merges parts of:
* PR microsoft#662 "Overhaul the build process for the OpenSSL dependency on
  Android" by jwtowner
* PR microsoft#711 "Update the build process for Boost on Android", also by
  jwtowner
* PR microsoft#714 "Improved the build process for cpprestsdk on Android", also
  by jwtowner.

The changes which required patching upstream Boost-for-Android in those
PRs has been stripped.

The NDK version targeted has been updated to 17c because that's what's
avaialble in Azure Pipelines.

Various compile failures this uncovered in cpprestsdk on Android were
fixed.

Author: BillyONeal

.gitignore

@@ -69,5 +69,5 @@ build.*/
 docs/
 # ignore NuGet artifacts
 .nuget/
-
-Generated Files/
+Build_android/build/
+Generated Files/

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "websocketspp"]
 	path = Release/libs/websocketpp
 	url = https://github.com/zaphoyd/websocketpp/
+[submodule "Boost-for-Android"]
+	path = Build_android/Boost-for-Android
+	url = https://github.com/moritz-wundke/Boost-for-Android