Update PYTHONPATH for adb.py restructuring.

Importers of the adb python package like lldbclient.py and gdbrunner
are broken following the restructure that enabled adb to export type
info to importers.

Bug: http://b/266688086
Test: adb.__file__ is no longer None after envsetup.sh/lunch
Change-Id: I7bf27601e800043adee8971c093347ca9b2979a9