Android: fix ownCloud lib build

The build relied on the presence of ANDROID_HOME envvar, now we pass
it explicitly.

Change-Id: If33ee2c6a22435151aad244378ee5aaf7b3cc913

external/owncloud-android-lib/ExternalProject_owncloud_android_lib.mk

@@ -18,6 +18,7 @@ $(call gb_ExternalProject_get_state_target,owncloud_android_lib,build) :
 	$(ICECREAM_RUN) "$(ANT)" \
 		-q \
 		-f build.xml \
+		-Dsdk.dir=$(ANDROID_SDK_HOME) \
 		release \
 	)