Warn if snapshots are enabled but not supported by the SDK Tools.

Supposedly snapshots (nowadays resurrected as "Quick Boot") should
work, but I haven't managed to get them to work via the command-line
tools, so until we figure that out, they'll remain disabled for recent
versions of the SDK Tools.

src/main/java/hudson/plugins/android_emulator/AndroidEmulator.java

@@ -350,6 +350,11 @@ private Environment doSetUp(final AbstractBuild<?, ?> build, final Launcher laun
         Proc adbStart2 = emu.getToolProcStarter(adbStartCmd).stdout(logger).stderr(logger).start();
         adbStart2.joinWithTimeout(5L, TimeUnit.SECONDS, listener);
 
+        // Show warning about snapshots being enabled, but not supported
+        if (useSnapshots && !androidSdk.supportsSnapshots()) {
+            log(logger, Messages.SNAPSHOTS_NOT_SUPPORTED());
+        }
+
         // Determine whether we need to create the first snapshot
         final SnapshotState snapshotState;
         if (useSnapshots && androidSdk.supportsSnapshots()) {

src/main/resources/hudson/plugins/android_emulator/Messages.properties

@@ -77,6 +77,7 @@ COULD_NOT_CREATE_EMULATOR=Could not create Android emulator: {0}
 SETTING_HARDWARE_PROPERTIES=Setting hardware properties:
 DELAYING_START_UP=Waiting {0} seconds before starting emulator...
 STARTING_EMULATOR=Starting Android emulator
+SNAPSHOTS_NOT_SUPPORTED=Snapshots are enabled, but cannot be used as they are not supported by the current SDK Tools
 STARTING_EMULATOR_FROM_SNAPSHOT=Starting Android emulator from snapshot
 STARTING_EMULATOR_SNAPSHOT_INIT=Starting Android emulator and creating initial snapshot
 ERASING_EXISTING_EMULATOR_DATA=Erasing existing emulator data...