releasetools: Ignore nonexistent APEX overrides.

This allows sharing the same signing config on different target_files
zips. Nonexistent APEX will be ignored with a warning.

Bug: 137249701
Test: Run sign_target_files_apks with APEX overrides.
Change-Id: I2bad0f5c00753ed36ec5ae3431c7dc2ff1fc3e9c
Merged-In: I2bad0f5c00753ed36ec5ae3431c7dc2ff1fc3e9c
(cherry picked from commit b369c722)