[AMS] Android 后台进程启动 activity 限制

https://developer.android.google.cn/guide/components/activities/background-starts?hl=lt

Android 10（API 级别 29）及更高版本对应用在后台运行时可以启动 activity 的时间施加了限制。这些限制有助于最大限度地减少对用户造成的干扰，并且可以让用户更好地控制其屏幕上显示的内容。

frameworks/base/services/core/java/com/android/server/wm/BackgroundActivityStartController.java

 boolean shouldAbortBackgroundActivityStart(
            int callingUid,
            int callingPid,
            final String callingPackage,
            int realCallingUid,
            int realCallingPid,
            WindowProcessController callerApp,
            PendingIntentRecord originatingPendingIntent,
            boolean allowBackgroundActivityStart,
            Intent intent,
            ActivityOptions checkedOptions) {
        return checkBackgroundActivityStart(callingUid, callingPid, callingPackage,
                realCallingUid, realCallingPid, callerApp, originatingPendingIntent,
                allowBackgroundActivityStart, intent, checkedOptions) == BAL_BLOCK;
    }
    /**
     * @return A code denoting which BAL rule allows an activity to be started,
     * or {@link BAL_BLOCK} if the launch should be blocked
     */
    @BalCode
    int checkBackgroundActivityStart(
            int callingUid,
            int callingPid,
            final String callingPackage,
            int realCallingUid,
            int realCallingPid,
            WindowProcessController callerApp,
            PendingIntentRecord originatingPendingIntent,
            boolean allowBackgroundActivityStart,
            Intent intent,
            ActivityOptions checkedOptions) {
        // don't abort for the most important UIDs
        final int callingAppId = UserHandle.getAppId(callingUid);
        final boolean useCallingUidState =
                originatingPendingIntent == null
                        || checkedOptions == null
                        || !checkedOptions.getIgnorePendingIntentCreatorForegroundState();
        if (useCallingUidState) {
            if (callingUid == Process.ROOT_UID
                    || callingAppId == Process.SYSTEM_UID
                    || callingAppId == Process.NFC_UID) {
                return logStartAllowedAndReturnCode(BAL_ALLOW_ALLOWLISTED_UID, /*background*/ false,
                        callingUid, realCallingUid, intent, "Important callingUid");
            }
            // Always allow home application to start activities.
            if (isHomeApp(callingUid, callingPackage)) {
                return logStartAllowedAndReturnCode(BAL_ALLOW_ALLOWLISTED_COMPONENT,
                        /*background*/ false, callingUid, realCallingUid, intent,
                        "Home app");
            }
            // IME should always be allowed to start activity, like IME settings.
            final WindowState imeWindow =
                    mService.mRootWindowContainer.getCurrentInputMethodWindow();
            if (imeWindow != null && callingAppId == imeWindow.mOwnerUid) {
                return logStartAllowedAndReturnCode(BAL_ALLOW_ALLOWLISTED_COMPONENT,
                        /*background*/ false, callingUid, realCallingUid, intent,
                        "Active ime");
            }
        }
        // This is used to block background activity launch even if the app is still
        // visible to user after user clicking home button.
        final int appSwitchState = mService.getBalAppSwitchesState();
        // don't abort if the callingUid has a visible window or is a persistent system process
        final int callingUidProcState = mService.mActiveUids.getUidState(callingUid);
        final boolean callingUidHasAnyVisibleWindow = mService.hasActiveVisibleWindow(callingUid);
        final boolean isCallingUidForeground =
                callingUidHasAnyVisibleWindow
                        || callingUidProcState == ActivityManager.PROCESS_STATE_TOP
                        || callingUidProcState == ActivityManager.PROCESS_STATE_BOUND_TOP;
        final boolean isCallingUidPersistentSystemProcess =
                callingUidProcState