android: raise minSdkVersion to 21

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

src/input/plugins/FileInputPlugin.cxx

@@ -65,12 +65,9 @@ OpenFileInputStream(Path path,
 		throw FormatRuntimeError("Not a regular file: %s",
 					 path.c_str());
 
-#if !defined(__BIONIC__) || __ANDROID_API__ >= 21
-	/* posix_fadvise() requires Android API 21 */
 #ifdef POSIX_FADV_SEQUENTIAL
 	posix_fadvise(reader.GetFD().Get(), (off_t)0, info.GetSize(),
 		      POSIX_FADV_SEQUENTIAL);
-#endif
 #endif
 
 	return InputStreamPtr(new FileInputStream(path.ToUTF8().c_str(),

src/system/EPollFD.cxx

@@ -22,21 +22,6 @@
 #include "EPollFD.hxx"
 #include "FatalError.hxx"
 
-#if defined(__BIONIC__) && __ANDROID_API__ < 21
-
-#include <sys/syscall.h>
-#include <fcntl.h>
-
-#define EPOLL_CLOEXEC O_CLOEXEC
-
-static inline int
-epoll_create1(int flags)
-{
-    return syscall(__NR_epoll_create1, flags);
-}
-
-#endif
-
 EPollFD::EPollFD()
 	:fd(::epoll_create1(EPOLL_CLOEXEC))
 {