[Bugfix]use walkFileTree to calucate apk size

When using foreach on the stream of Files.walk, the FileTreeIterator may generate an UncheckedIOException, which can cause a system crash.

Bug:292486804
Test:
1.statsd_testdrive 524
2.adb install any apk
3.observe apks_size_bytes

Merged-In: I18852ced54ceeb63785033de3f1ab22a35ba1fc1
Change-Id: I0e223fc61184a51a4f5a71df54ba50137376df8e