Sanity-check erroneous backup agent instantiations

Two distinct changes:

Fix a bug seen in the wild where a newly-launched application will be
spuriously asked to instantiate a backup agent.  What was happening
there is that some Activity Manager state was being left stale in certain
circumstances, and then in combination with app uninstall / install, there
could be a case where uid reuse wound up looking like an app identity
match.

We now positively verify before instantiating the agent that the intended
backup target package is uid-compatible with the app process that the
instantiation was requested of.  The incomplete bookkeeping in the
Activity Manager has also been tightened up, and the Backup Manager is
more aggressive about cleaning up pending operations pertaining to
apps being uninstalled.

Bug 5874010

Change-Id: Ic389f4a96c9dcd0ba6b3962b579084033d8ae9f8

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

@@ -1474,6 +1474,7 @@ class BackupManagerService extends IBackupManager.Stub {
             if (MORE_DEBUG) Slog.v(TAG, "  removing participant " + packageName);
             removeEverBackedUp(packageName);
             set.remove(packageName);
+            mPendingBackups.remove(packageName);
         }
     }
 
@@ -1625,6 +1626,7 @@ class BackupManagerService extends IBackupManager.Stub {
                         } catch (InterruptedException e) {
                             // just bail
                             if (DEBUG) Slog.w(TAG, "Interrupted: " + e);
+                            mActivityManager.clearPendingBackup();
                             return null;
                         }
                     }
@@ -1632,6 +1634,7 @@ class BackupManagerService extends IBackupManager.Stub {
                     // if we timed out with no connect, abort and move on
                     if (mConnecting == true) {
                         Slog.w(TAG, "Timeout waiting for agent " + app);
+                        mActivityManager.clearPendingBackup();
                         return null;
                     }
                     if (DEBUG) Slog.i(TAG, "got agent " + mConnectedAgent);