Parses accessibility packages without holding A11yManagerService#mLock.

Package resource parsing can randomly take a very long time, so we
shouldn't hold the singular A11y mLock while doing this parsing or else
unrelated threads will be blocked.

Introduces a new flag:
- namespace: accessibility
- flag: com.android.server.accessibility.scan_packages_without_lock

Bug: 295969873
Test: Enable/disable the flag. Observe unchanged behavior when
      installing and uninstalling a11y packages and switching users.
Test: atest AccessibilityManagerServiceTest
Change-Id: I1bc05403c14bfdd8da7c6621048d93df98f1ddf5
diff --git a/services/accessibility/accessibility.aconfig b/services/accessibility/accessibility.aconfig
index 10ac2eb..25b6244 100644
--- a/services/accessibility/accessibility.aconfig
+++ b/services/accessibility/accessibility.aconfig
@@ -48,3 +48,10 @@
     description: "Stops using the deprecated PackageListObserver."
     bug: "304561459"
 }
+
+flag {
+    name: "scan_packages_without_lock"
+    namespace: "accessibility"
+    description: "Scans packages for accessibility service/activity info without holding the A11yMS lock"
+    bug: "295969873"
+}