Fix the build failure with enabling DEBUG_PSS

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java:2688:
error: variable pid might not have been initialized
    if (DEBUG_PSS) Slog.d(TAG_PSS, "Skipped pss collection of " + pid +

Bug: None
Test: make -j12
Change-Id: Ib668ba2606348f92239355b04039b45cae025cf3