android: add CI jobs, buildinfo, cmake docs, disable CURL_USE_PKGCONFIG by default

- GHA/non-native: add Android builds, both cmake and autotools,
  both NDK 21 (oldest available) and 35 (newest available)
  https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md
  It comes with a maintenance burden to bump the oldest/latest values
  with CI runner updates.

- cmake: disable `CURL_USE_PKGCONFIG` by default for Android.
  To avoid picking up system package by default.

- build: add `ANDROID-<NDK-LEVEL>` flag to `buildinfo.txt`.
  Also detect NDK level with the CMake built-in build method:
  https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-android

- INSTALL.md: add CMake build instructions for Android.

- INSTALL.md: make NDK levels consistent in `./configure` example.

Closes #16014

acinclude.m4

@@ -1539,6 +1539,15 @@ AC_DEFUN([CURL_PREPARE_BUILDINFO], [
   if test "$curl_cv_winuwp" = 'yes'; then
     curl_pflags="${curl_pflags} UWP"
   fi
+  case $host in
+    *-*-android*)
+      curl_pflags="${curl_pflags} ANDROID"
+      ANDROID_PLATFORM_LEVEL=`echo "$host_os" | $SED -ne 's/.*android\(@<:@0-9@:>@*\).*/\1/p'`
+      if test -n "${ANDROID_PLATFORM_LEVEL}"; then
+        curl_pflags="${curl_pflags}-${ANDROID_PLATFORM_LEVEL}"
+      fi
+      ;;
+  esac
   if test "$curl_cv_cygwin" = 'yes'; then
     curl_pflags="${curl_pflags} CYGWIN"
   fi