Add MiniFence to drop HWC2on1Adapter libui dep

This class will soon become a library used by devices with no support
for HWC2. They will link against libhwc2on1adapter. Drivers should have
no depedencies on libui.

Test:Manual
Change-Id: Iabd2aa89fc3b737a999632a16c4f6c30464787c4
diff --git a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
index 4187890..13bf0b5 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2On1Adapter.cpp
@@ -724,7 +724,7 @@
         }
 
         auto releaseFence = layer->getReleaseFence();
-        if (releaseFence != Fence::NO_FENCE) {
+        if (releaseFence != MiniFence::NO_FENCE) {
             if (outputsNonNull) {
                 outLayers[numWritten] = layer->getId();
                 outFences[numWritten] = releaseFence->dup();
@@ -2003,7 +2003,7 @@
     mReleaseFence.add(fenceFd);
 }
 
-const sp<Fence>& HWC2On1Adapter::Layer::getReleaseFence() const {
+const sp<MiniFence>& HWC2On1Adapter::Layer::getReleaseFence() const {
     return mReleaseFence.get();
 }