Android support (#244)

Add Android support

### Motivation:

Support the Android platform.

### Modifications:

Add Android imports and fix the default temporary directory to be
correct for the OS.

### Result:

The package will build and test on Android.

---------

Co-authored-by: George Barnett <gbarnett@apple.com>

Sources/NIOExtras/DebugInboundEventsHandler.swift

@@ -17,6 +17,8 @@ import NIOCore
 import Darwin
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(Android)
+import Android
 #else
 import Glibc
 #endif

Sources/NIOExtras/DebugOutboundEventsHandler.swift

@@ -18,6 +18,8 @@ import NIOCore
 import Darwin
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(Android)
+import Android
 #else
 import Glibc
 #endif

Sources/NIOExtras/WritePCAPHandler.swift

@@ -20,6 +20,8 @@ import NIOCore
 import Darwin
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(Android)
+import Android
 #else
 import Glibc
 #endif

Sources/NIOSOCKS/Messages/SOCKSRequest.swift

@@ -19,6 +19,8 @@ import NIOCore
 import Darwin
 #elseif canImport(Musl)
 import Musl
+#elseif canImport(Android)
+import Android
 #else
 import Glibc
 #endif

Tests/NIOExtrasTests/SynchronizedFileSinkTests.swift

@@ -108,6 +108,8 @@ private func withTemporaryFile<T>(
 private var temporaryDirectory: String {
     #if os(Linux)
     return "/tmp"
+    #elseif os(Android)
+    return "/data/local/tmp"
     #else
     if #available(macOS 10.12, iOS 10, tvOS 10, watchOS 3, *) {
         return FileManager.default.temporaryDirectory.path

Tests/NIOExtrasTests/WritePCAPHandlerTest.swift

@@ -19,6 +19,10 @@ import XCTest
 
 @testable import NIOExtras
 
+#if canImport(Android)
+import Android
+#endif
+
 class WritePCAPHandlerTest: XCTestCase {
     private var accumulatedPackets: [ByteBuffer]!
     private var channel: EmbeddedChannel!