API CHANGE: expose backup/restore to the SDK

The core backup/restore classes [BackupManager, BackupAgent, RestoreSession, and
RestoreObserver] are now published for 3rd party developers, as well as the suite
of helper classes that exist so far to aid authorship of backup/restore agents.

In conjunction with the API change, the restore-time automatic data wipe has now
been removed:  applications are responsible for managing the logic of wipe vs
merge themselves.  If the app's agent onRestore() callback throws, the data
is presumed to be incoherent and a wipe is issued via the Activity Manager;
otherwise, no automatic action is ever taken.

Change-Id: I0b3418b829d4689b58b88be3d9c4ace37a8583a9

services/java/com/android/server/BackupManagerService.java

@@ -1083,15 +1083,11 @@ class BackupManagerService extends IBackupManager.Stub {
 
         synchronized(mClearDataLock) {
             mClearingData = true;
-            /* This is causing some critical processes to be killed during setup.
-               Temporarily revert this change until we find a better solution.
             try {
                 mActivityManager.clearApplicationUserData(packageName, observer);
             } catch (RemoteException e) {
                 // can't happen because the activity manager is in this process
             }
-            */
-            mPackageManager.clearApplicationUserData(packageName, observer);
 
             // only wait 10 seconds for the clear data to happen
             long timeoutMark = System.currentTimeMillis() + TIMEOUT_INTERVAL;
@@ -1660,10 +1656,6 @@ class BackupManagerService extends IBackupManager.Stub {
                             + "] is compatible with installed version ["
                             + packageInfo.versionCode + "]");
 
-                    // Now perform the actual restore:  first clear the app's data
-                    // if appropriate
-                    clearApplicationDataSynchronous(packageName);
-
                     // Then set up and bind the agent (with a restricted Application object
                     // unless the application says otherwise)
                     boolean useRealApp = (packageInfo.applicationInfo.flags