use directory timestamp during OTA

Previously the timestamp was generated using the directory and not the
APK [see b/29575840 and b/29610181 as to why that's bad]. In MR1, the
timestamp was generated using the most recent time of any of the APK
files.

The timestamp discrepancy causes problems during OTA to N MR1. Most
apps are incorrectly marked as "changed" which means a full parse and
verification pass on all packages. This can be a very lengthy process.

Bug: 30930797
Change-Id: Iaef7b318e1c1effbd74d996f88cbf3d6e77736cd
Test: manually updated the platform and noticed that existing packages were marked as "not changed"