PackageUtil: don't collect certs if package doesn't exist

If packageParser.parsePackage() returns null, don't attempt call
packageParser.collectCertificates().

This fixes a NPE triggered by the following command:

am start -a android.intent.action.VIEW -d file:///blah/blah.apk -t application/vnd.android.package-archive

Change-Id: I6f6f836734d4b4ee07e98ee6b1f6339fa675b8b0

Conflicts:
	src/com/android/packageinstaller/PackageUtil.java
Move TabsAdapter into it's own class.

Extract TabsAdapter into it's own class.  It makes the code
more reusable and makes PackageInstallerActivity.java smaller.
No change in functionality.

Change-Id: I1ebc7fd05f600fd4764020c5ab72274f435ed211

Conflicts:
	src/com/android/packageinstaller/PackageInstallerActivity.java
	src/com/android/packageinstaller/TabsAdapter.java
Use direct call to get ManifestDigest

This speeds up startup of PackageInstaller since it doesn't have to read
all the data.

Bug: 8528639
Change-Id: Ic7b9dc93f0f940a128b583936b3f56c707b5cb2c
PackageInstaller: Show current & new version

Change-Id: Iee10d988c095475e1ccbcea7a9c64d9e114d79e6

Conflicts:
	src/com/android/packageinstaller/PackageInstallerActivity.java