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