Allow each APEX to provide its own AndroidManifest

AndroidManifest.xml file can be specified via the 'androidManifest'
property in the apex module type. It can be used to have a custom
AndroidManifest.xml that have additional tags (such as <uses-sdk> or
<uses-feature>) for precise targeting.

The property is optional; if unspecified, then one is auto-generated as
before.

Bug: 123857186
Test: m apex_test_build_features
Change-Id: Id7ee0471661887cfa11124cbaa3beea8cffcdda4