gg: fixes and Android support (#6161)

vlib/os/os_android.c.v

@@ -0,0 +1,30 @@
+module os
+
+struct C.ANativeActivity {
+	assetManager &C.AAsetManager
+}
+
+struct C.AAset {}
+
+fn C.AAssetManager_open(&C.AAsetManager, charptr, int) &C.AAset
+fn C.AAsset_getLength(&C.AAset) int
+fn C.AAsset_read(&C.AAset, voidptr, int) int
+fn C.AAsset_close(&C.AAsset)
+
+pub fn read_apk_asset(file string) ?[]byte {
+	act := &C.ANativeActivity(C.sapp_android_get_native_activity())
+	if isnil(act) {
+		return error('Could not get reference to Android activity')
+	}
+	asset := C.AAssetManager_open(act.assetManager, file.str, C.AASSET_MODE_STREAMING)
+	if isnil(asset) {
+		return error('File `$file` not found')
+	}
+	len := C.AAsset_getLength(asset)
+	buf := []byte{ len: len }
+	for {
+		if C.AAsset_read(asset, buf.data, len) > 0 { break }
+	}
+	C.AAsset_close(asset)
+	return buf
+}