[build] Use API-21 for all native builds

Up until this commit, we have been building the 64-bit targets (`x86-64`
and `armv8-a` against NDK API 21 while the 32-bit targets (`x86` and
`armv7-a`) against NDK API 16.  This commit changes it so that all
targets build against API 21.  This gives us the ability to speed up
Debug builds on 32-bit platforms via use of  POSIX APIs for directory
traversal that are available starting from API 21 onwards (`openat` and
`fstatat`).  The switch to API 21 was initially planned for .NET5 but
there's no harm in making the change now.

Author: grendello

build-tools/xaprepare/xaprepare/ConfigAndData/BuildAndroidPlatforms.cs

@@ -42,9 +42,9 @@ class BuildAndroidPlatforms
 		};
 
 		public static readonly Dictionary<string, uint> NdkMinimumAPI = new Dictionary<string, uint> {
-			{ AbiNames.TargetJit.AndroidArmV7a, 16 },
+			{ AbiNames.TargetJit.AndroidArmV7a, 21 },
 			{ AbiNames.TargetJit.AndroidArmV8a, 21 },
-			{ AbiNames.TargetJit.AndroidX86,    16 },
+			{ AbiNames.TargetJit.AndroidX86,    21 },
 			{ AbiNames.TargetJit.AndroidX86_64, 21 },
 		};
 	}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/NdkUtilTests.cs

@@ -41,7 +41,7 @@ public void TestNdkUtil ()
 				Assert.IsTrue (NdkUtil.ValidateNdkPlatform (log, ndkDir, arch, enableLLVM: false));
 				Assert.AreEqual (0, errors.Count, "NdkUtil.ValidateNdkPlatform should not have returned false.");
 				int level = NdkUtil.GetMinimumApiLevelFor (arch, ndkDir);
-				int expected = 16;
+				int expected = 21;
 				Assert.AreEqual (expected, level, $"Min Api Level for {arch} should be {expected}.");
 				var compilerNoQuotes = NdkUtil.GetNdkTool (ndkDir, arch, "gcc", level);
 				Assert.AreEqual (0, errors.Count, "NdkUtil.GetNdkTool should not have errored.");

src/monodroid/jni/embedded-assemblies.cc

@@ -583,7 +583,7 @@ EmbeddedAssemblies::typemap_read_header ([[maybe_unused]] int dir_fd, const char
 	struct stat sbuf;
 	int res;
 
-#if __ANDROID_API__ < 21
+#if defined (WINDOWS)
 	simple_pointer_guard<char[]> full_file_path = utils.path_combine (dir_path, file_path);
 	res = stat (full_file_path, &sbuf);
 #else
@@ -600,7 +600,7 @@ EmbeddedAssemblies::typemap_read_header ([[maybe_unused]] int dir_fd, const char
 		return false;
 	}
 
-#if __ANDROID_API__ < 21
+#if defined (WINDOWS)
 	fd = open (full_file_path, O_RDONLY);
 #else
 	fd = openat (dir_fd, file_path, O_RDONLY);
@@ -800,7 +800,7 @@ EmbeddedAssemblies::try_load_typemaps_from_directory (const char *path)
 	}
 
 	int dir_fd;
-#if __ANDROID_API__ < 21
+#if WINDOWS
 	dir_fd = -1;
 #else
 	dir_fd = dirfd (dir);