Add manifestPlaceholders feature

[misl6]

Needs kivy/python-for-android#2301 to be merged.

This PR adds the support for manifestPlaceholders feature that's it's now implemented in python-for-android

[AndreMiras]

Thanks for the changes.
Could you look into the failing integration test?

2020-08-19T16:44:44.3292064Z usage: toolchain.py [-h] [--private PRIVATE] --package PACKAGE --name NAME
2020-08-19T16:44:44.3292703Z                     [--numeric-version NUMERIC_VERSION] --version VERSION
2020-08-19T16:44:44.3293195Z                     [--launcher] [--permission PERMISSIONS [PERMISSIONS ...]]
2020-08-19T16:44:44.3293647Z                     [--meta-data META_DATA] [--uses-library ANDROID_USED_LIBS]
2020-08-19T16:44:44.3294088Z                     [--asset /path/to/source:dest] [--icon ICON]
2020-08-19T16:44:44.3294526Z                     [--service SERVICES] [--native-service NATIVE_SERVICES]
2020-08-19T16:44:44.3294952Z                     [--presplash PRESPLASH]
2020-08-19T16:44:44.3295376Z                     [--presplash-color PRESPLASH_COLOR] [--window]
2020-08-19T16:44:44.3295771Z                     [--orientation ORIENTATION]
2020-08-19T16:44:44.3296203Z                     [--android-entrypoint ANDROID_ENTRYPOINT]
2020-08-19T16:44:44.3296603Z                     [--android-apptheme ANDROID_APPTHEME]
2020-08-19T16:44:44.3297012Z                     [--add-compile-option COMPILE_OPTIONS]
2020-08-19T16:44:44.3297434Z                     [--add-gradle-repository GRADLE_REPOSITORIES]
2020-08-19T16:44:44.3297878Z                     [--add-packaging-option PACKAGING_OPTIONS] [--wakelock]
2020-08-19T16:44:44.3298380Z                     [--blacklist BLACKLIST] [--whitelist WHITELIST]
2020-08-19T16:44:44.3298811Z                     [--release] [--with-debug-symbols] [--add-jar ADD_JAR]
2020-08-19T16:44:44.3299255Z                     [--add-aar ADD_AAR] [--depend DEPENDS] [--sdk SDK_VERSION]
2020-08-19T16:44:44.3299663Z                     [--minsdk MIN_SDK_VERSION]
2020-08-19T16:44:44.3300065Z                     [--allow-minsdk-ndkapi-mismatch]
2020-08-19T16:44:44.3300479Z                     [--intent-filters INTENT_FILTERS]
2020-08-19T16:44:44.3300884Z                     [--with-billing BILLING_PUBKEY]
2020-08-19T16:44:44.3301283Z                     [--add-source EXTRA_SOURCE_DIRS]
2020-08-19T16:44:44.3301716Z                     [--try-system-python-compile] [--no-compile-pyo] [--sign]
2020-08-19T16:44:44.3302105Z                     [--add-activity ADD_ACTIVITY]
2020-08-19T16:44:44.3302540Z                     [--activity-launch-mode ACTIVITY_LAUNCH_MODE]
2020-08-19T16:44:44.3302950Z                     [--allow-backup ALLOW_BACKUP] [--no-optimize-python]
2020-08-19T16:44:44.3303369Z                     [--extra-manifest-xml EXTRA_MANIFEST_XML]
2020-08-19T16:44:44.3303782Z toolchain.py: error: unrecognized arguments: --manifest-placeholders [:]
2020-08-19T16:44:44.3515704Z # Command failed: /opt/hostedtoolcache/Python/3.8.5/x64/bin/python -m pythonforandroid.toolchain apk --debug --bootstrap=sdl2 --dist_name myapp --name 'My Application' --version 0.1 --package org.test.myapp --minsdk 21 --ndk-api 21 --private /home/runner/work/buildozer/buildozer/.buildozer/android/app --android-entrypoint org.kivy.android.PythonActivity --android-apptheme @android:style/Theme.NoTitleBar --orientation portrait --window --copy-libs --manifest-placeholders [:] --arch armeabi-v7a --color=always --storage-dir="/home/runner/work/buildozer/buildozer/.buildozer/android/platform/build-armeabi-v7a" --ndk-api=21

Edit, OK I realise you addressed it in kivy/python-for-android#2301

[misl6]

@AndreMiras Yes, the tests are not going to be green until kivy/python-for-android#2301 is merged into p4a/develop .

I'm going to force the tests again, after kivy/python-for-android#2301 it's merged.

[AndreMiras]

Now I realise default p4a.branch is master so yes that doesn't work either after merge. We probably want p4a.branch = develop for integration tests I guess. Fix shouldn't be that hard I think.
Maybe you could try something similar to https://github.com/kivy/buildozer/blob/020a550/.github/workflows/android.yml#L25

[AndreMiras]

LGTM, thanks for the follow-up and rebasing.
I've added a minor question

[inclement]

This is causing buildozer to fail for people not using python-for-android master. I think we should revert and make this option only do something if a value is explicitly set by the user.